High Yield Overview

TKA Balancing and Instability

Diagnose the gap | correct the cause | use the least necessary constraint

0° / 30° / 90°Stress positions to examine

Flexion gap largerCore flexion-instability pattern

CR → PS → VVC → hingeConstraint ladder

PJI firstExclude infection before revision

Instability Pattern

Flexion instability

PatternFlexion gap is larger than extension gap.

TreatmentRestore posterior offset, reduce tibial slope, revise PCL strategy and increase constraint if required.

Extension instability

PatternExtension gap is larger than flexion gap.

TreatmentRestore distal femoral joint line, use distal augments and avoid isolated insert exchange unless both gaps are loose.

Mid-flexion instability

PatternStable at 0° and 90° but unstable around 30° to 60°.

TreatmentLook for joint-line elevation, component design/position, collateral laxity and malrotation.

Global or coronal instability

PatternCollateral incompetence, malalignment, bone loss or multiplanar laxity.

TreatmentRevise the cause and choose VVC/CCK or hinge according to residual soft-tissue competence.

Critical Must-Knows

A balanced TKA is not simply a thick insert. It is stable in extension, flexion and mid-flexion with a centred patella and no coronal thrust.
Distal femoral resection changes extension gap. Posterior femoral condylar resection and posterior tibial slope mainly change flexion gap.
Flexion instability often presents with stairs, chair-rise difficulty, recurrent effusions and vague anterior/periretinacular pain.
Mid-flexion instability is a real but difficult diagnosis. It requires careful examination between full extension and 90° rather than a single endpoint test.
Constraint treats residual instability after the cause is corrected. It does not compensate for malrotation, malalignment, infection or unrecognised bone loss.

Clinical Pearls

"
Varus-valgus stress must be tested at full extension, 30° and 90° because each position loads different stabilisers.
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A thicker polyethylene insert tightens flexion and extension together; it cannot selectively fix a flexion gap problem without risking stiffness or overstuffing.
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In a CR TKA, PCL incompetence is a classic cause of flexion instability; conversion to PS may be enough only if collaterals and component position are acceptable.
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In revision, reconstruct the platform first: infection status, fixation, component position, bone loss, joint line, posterior offset and ligament competence.

Do not treat instability by label alone

The question is not “flexion instability equals PS” or “global instability equals hinge”. The surgeon must decide whether the cause is infection, component malposition, PCL failure, collateral incompetence, joint-line elevation, bone loss, posterior offset loss, tibial slope, extensor mechanism failure or true global laxity.

TKA gap balancing map — The treatment follows the gap. A flexion-loose knee needs a different correction from an extension-loose knee, and an asymmetric gap needs selective release or component correction rather than simply a thicker insert.Credit: Original OrthoVellum illustration

At a Glance: The Gap Diagnosis

Finding	Likely Problem	Correction Principle
Loose at 90° but stable in extension	Flexion gap too large.	Increase posterior femoral offset, reduce excessive slope, correct PCL strategy, revise malrotation if present.
Loose in extension but acceptable at 90°	Extension gap too large.	Restore distal femoral joint line with distal augments; do not rely on thicker insert alone.
Loose at 30° to 60° but stable at endpoints	Mid-flexion instability.	Assess joint line, femoral component design/position, collateral isometry and coronal laxity.
Varus or valgus opening at multiple flexion angles	Collateral incompetence, asymmetric release, malalignment or bone loss.	Correct alignment/bone loss and use VVC/CCK or hinge if soft tissues cannot stabilise the knee.
Painful instability with raised inflammatory markers	Possible periprosthetic joint infection.	Investigate infection before any instability revision.

Core Mental Models

D-P-SWhat changes gaps	PJIBefore revision	CPVHConstraint ladder
D Distal femur Mainly changes extension gap.	P Pain pattern Pain may be infection, loosening, malrotation, instability or extensor failure.	C CR PCL and collaterals competent.
P Posterior condyles Mainly changes flexion gap and posterior offset.	J Joint aspiration Use when infection suspicion persists after blood tests.	P PS PCL deficient but collaterals competent.
S Slope More posterior tibial slope opens flexion gap, especially in CR knees.	I Implant position Do not revise instability without assessing component position.	V VVC/CCK Coronal laxity with usable extensor mechanism.
		H Hinge Global instability or severe bone/ligament loss.
Distal, posterior, slope.	Painful loose TKA: infection first.	Constraint rises only when soft-tissue control falls.

D-P-SWhat changes gaps

Distal femur

Mainly changes extension gap.

Posterior condyles

Mainly changes flexion gap and posterior offset.

Slope

More posterior tibial slope opens flexion gap, especially in CR knees.

Distal, posterior, slope.

PJIBefore revision

Pain pattern

Pain may be infection, loosening, malrotation, instability or extensor failure.

Joint aspiration

Use when infection suspicion persists after blood tests.

Implant position

Do not revise instability without assessing component position.

Painful loose TKA: infection first.

CPVHConstraint ladder

PCL and collaterals competent.

PCL deficient but collaterals competent.

VVC/CCK

Coronal laxity with usable extensor mechanism.

Hinge

Global instability or severe bone/ligament loss.

Constraint rises only when soft-tissue control falls.

Mnemonic

GAPSSafe Revision Sequence

Gap diagnosis

Decide flexion, extension, mid-flexion, coronal or global instability.

Alignment and infection

Exclude PJI and assess mechanical axis, component fixation and rotation.

Posterior offset and slope

Correct the common drivers of flexion instability.

Soft tissues and constraint

Use the least constraint that controls the corrected knee.

Memory Hook:GAPS keeps the revision plan cause-based rather than insert-based.

Overview and Definitions

TKA balancing means creating a knee that is stable through the full arc of motion without excessive constraint, stiffness or asymmetric soft-tissue tension. It is a bone, ligament, implant and alignment problem at the same time.

The key definitions are practical:

Terms That Matter

Term	Meaning	Why It Matters
Extension gap	Space and soft-tissue tension with the knee at full extension.	Affected mainly by distal femoral resection, posterior capsule, collateral tension and osteophytes.
Flexion gap	Space and soft-tissue tension at 90° flexion.	Affected mainly by posterior condylar resection, femoral sizing, posterior offset, tibial slope and PCL status.
Rectangular gap	Medial and lateral compartments open evenly under tension.	Needed for coronal stability and central tracking.
Asymmetric gap	One compartment remains tighter or looser than the other.	Requires selective release, recut, component correction or constraint depending cause.
Constraint	Mechanical stability built into the implant.	Useful when soft tissues cannot stabilise the knee, but it increases load transfer to fixation interfaces.

Surgically Relevant Anatomy

The knee is stabilised by bone cuts, implant geometry and soft tissues. A balanced TKA depends on respecting how each structure behaves in extension, flexion and mid-flexion.

Structures and Their Balancing Role

Structure	Main Role	Operative Relevance
Superficial MCL	Primary medial restraint, especially against valgus stress.	Over-release in varus TKA causes medial instability and may force higher constraint.
Deep MCL and posteromedial capsule	Medial tightness near the joint line.	Often released early after osteophyte removal in varus knees.
PCL	Posterior stabiliser and flexion-gap restraint in CR TKA.	Incompetence creates flexion instability; excessive tension can limit flexion or lift off.
LCL and posterolateral structures	Lateral stability, especially in valgus knees.	Release sequence depends on whether tightness is in extension, flexion or both.
IT band	Lateral extension tightness.	Release if valgus knee is tight mainly in extension.
Popliteus	Posterolateral flexion tightness.	Release cautiously if valgus knee is tight mainly in flexion.
Posterior osteophytes and capsule	Block extension and distort gap assessment.	Remove osteophytes before judging true extension balance.
Extensor mechanism	Patellar tracking and active extension.	Extensor failure can mimic instability and changes reconstruction choices.

Pathophysiology

Instability occurs when the soft-tissue envelope and implant geometry no longer control the tibia under load. The mechanism is usually mechanical, but pain and swelling from infection or loosening can make the knee feel unstable; this is why infection and fixation are checked before revising gaps.

Mechanism to Failure Pattern

Mechanism	What Happens Biomechanically	Clinical Pattern
Posterior condylar offset loss	Flexion gap opens because the femur is effectively undersized posteriorly.	Giving way in flexion, stairs difficulty and AP drawer at 90°.
Excess posterior tibial slope	Tibia rests in a posteriorly sloped platform, increasing flexion laxity.	Flexion instability, especially in CR TKA with PCL insufficiency.
Excess distal femoral resection	Extension gap opens more than flexion gap.	Recurvatum tendency or instability in stance.
Femoral or tibial malrotation	Flexion gap becomes asymmetric and patellofemoral tracking may fail.	Pain, stiffness, patellar symptoms or asymmetric flexion instability.
Collateral incompetence	The knee cannot resist varus-valgus load despite acceptable bone cuts.	Coronal opening at 0°, 30° or 90°; may require VVC/CCK or hinge.
Joint-line elevation	Collateral isometry and patellofemoral mechanics change through mid-flexion.	Possible mid-flexion instability, stiffness or extensor symptoms.

Primary TKA Balancing

Primary TKA varus and valgus soft tissue balancing sequence — Soft-tissue balancing should be stepwise. Remove osteophytes first, release the structure that is tight, then reassess at 0°, 30° and 90° before proceeding.Credit: Original OrthoVellum illustration

Before releases

Do not release soft tissue until the basics are correct:

Confirm the tibial cut is not in unintended varus or valgus.
Confirm distal femoral resection and femoral rotation are appropriate.
Remove posterior, medial and lateral osteophytes.
Clear meniscal remnants and loose bodies that tent the collateral ligaments.
Test with spacer blocks or trials at extension, 30° and 90°.
Correct obvious bone-cut error before sacrificing ligament.

Posterior compartment osteophyte access in TKA gap balancing — Posterior osteophytes can falsely tighten the extension gap. This open-access operative image is used only to illustrate posterior-compartment access in gap-balancing technique.Credit: Daines BK and Dennis DA via Clinics in Orthopedic Surgery / Open-i (CC BY)

Varus knee release sequence

Varus Knee: Medial Tightness

Step	Action	Decision Point
1	Remove medial tibial and femoral osteophytes.	If this corrects the gap, stop releasing.
2	Release deep MCL and posteromedial capsule from proximal tibia.	Good first soft-tissue step for most fixed varus knees.
3	Release semimembranosus if posteromedial tightness persists.	Useful when extension and posteromedial flexion remain tight.
4	Pie-crust or cautiously lengthen superficial MCL if severe medial tightness remains.	Risk is medial instability; reassess repeatedly.
5	Consider reduction osteotomy, sliding medial epicondyle osteotomy or constraint for severe fixed deformity.	Do this instead of uncontrolled MCL destruction.

Valgus knee release sequence

Valgus Knee: Lateral Tightness

Tightness Pattern	Likely Structure	Surgical Response
Tight mainly in extension	IT band and posterolateral capsule.	Release extension-side lateral structures first and reassess.
Tight mainly in flexion	Popliteus or LCL depending pattern.	Release flexion-side structure cautiously; avoid creating lateral flexion laxity.
Tight in flexion and extension	LCL and broader posterolateral complex.	Sequential release; high chance of needing increased constraint.
Medial side incompetent	Attenuated MCL in severe valgus.	Do not rely on soft-tissue release alone; consider VVC/CCK or hinge.

Clinical Assessment of the Unstable TKA

Painful or unstable TKA diagnostic sequence — The diagnostic sequence begins with symptoms and examination, then excludes infection, assesses implant position and classifies the instability pattern before choosing treatment.Credit: Original OrthoVellum illustration

History

Ask when the knee gives way. Flexion instability is classically worse with stairs, descending slopes, rising from a chair and activities that load the knee in flexion. Extension instability or recurvatum is more obvious during stance and level walking. Mid-flexion instability may feel vague: “not trusting the knee” between full extension and deep flexion.

Key history points:

Timing after primary TKA: immediate, early, late or after trauma.
Pain first or instability first.
Recurrent effusions or swelling after activity.
Stairs, chair rise, pivoting, level walking and downhill walking.
Previous infection, wound issues, manipulation, stiffness or revision.
Implant type if known: CR, PS, constrained condylar, rotating hinge.
Neuromuscular disease, extensor mechanism symptoms or falls.

Examination technique

How To Examine Instability

Test	How To Perform It	Positive Meaning
Gait observation	Watch walking from front, side and back; look for varus/valgus thrust, recurvatum, limp and extensor lag.	Thrust suggests coronal instability or malalignment; recurvatum suggests extension instability or neuromuscular control problem.
Varus-valgus stress at 0°	Patient supine, knee fully extended, stabilise femur and apply varus/valgus force at ankle.	Opening in extension suggests collateral incompetence, excessive extension gap or global instability.
Varus-valgus stress at 30°	Unlock the knee slightly and repeat stress testing.	Isolates collateral behaviour better than full extension; useful for mid-flexion/coronal laxity.
Varus-valgus stress at 90°	Knee flexed to 90°, foot supported, stress through the tibia.	Opening in flexion suggests flexion-gap asymmetry or collateral incompetence in flexion.
AP drawer at 90°	Stabilise foot, flex knee to 90°, translate tibia anterior and posterior.	Excess AP translation suggests flexion instability, PCL failure in CR TKA or post-cam problems in PS TKA.
Mid-flexion stress	Test coronal stability around 30° to 60° with the patient relaxed.	Instability here with stable endpoints supports mid-flexion instability.
Extensor mechanism	Assess active straight-leg raise, extensor lag, patellar tracking and apprehension.	Extensor failure or patellar instability can mimic giving way and changes reconstruction plan.

Investigations

What to request

Standing AP, lateral and skyline radiographs of the knee.
Full-length standing hip-to-ankle alignment radiograph.
Comparison with prior radiographs to assess subsidence, loosening and joint-line change.
Stress radiographs when clinical laxity needs documentation.
CT rotational profile for suspected femoral or tibial component malrotation.
ESR, CRP and aspiration when infection is possible.
Consider bone scan or SPECT/CT selectively when loosening is unclear and infection has been addressed.

What to measure

Imaging Checklist

Measurement	How To Assess	Why It Changes Treatment
Mechanical alignment	Full-length standing film: femoral head centre to ankle centre through knee.	Malalignment may drive coronal thrust and demands component or osteotomy-level correction.
Component coronal position	AP knee and long-leg films.	Varus tibial or valgus/varus femoral error can create asymmetric gaps.
Tibial posterior slope	True lateral radiograph.	Excess slope increases flexion gap, especially in CR TKA.
Posterior condylar offset	Lateral radiograph and comparison to pre-op/early post-op if available.	Loss of offset opens flexion gap and can cause flexion instability.
Joint-line elevation	Compare with fibular head, adductor tubercle or prior films.	Elevation can contribute to mid-flexion instability, patellar problems and extensor dysfunction.
Component rotation	CT using recognised femoral and tibial rotational references.	Malrotation can cause flexion-gap asymmetry, patellar maltracking, stiffness and pain.
Fixation and bone loss	Radiolucent lines, migration, osteolysis, CT if needed.	Loose or deficient components need revision reconstruction, not soft-tissue-only treatment.

Management Principles

When Non-Operative Care Is Reasonable

Situation	Treatment	Limit
Mild subjective symptoms, stable implant, no progressive laxity	Physiotherapy for quadriceps/hip strength, gait retraining and proprioception.	Does not correct mechanical gap mismatch.
Low-demand patient with mild coronal laxity and no loosening	Functional brace and activity modification.	Temporary or palliative option; monitor progression.
Early flexion symptoms without objective gross laxity	Strengthening, swelling control and careful review.	Persistent recurrent effusion or objective drawer needs surgical planning.
Infection suspected	Do not start definitive instability treatment until infection pathway is complete.	Missed infection makes revision fail.

When isolated polyethylene exchange is unsafe

Isolated insert exchange is unsafe when instability is due to malrotation, malalignment, loose components, component undersizing, bone loss, PCL failure with a CR design, collateral incompetence, extensor mechanism failure or infection. It may be considered only when components are well-fixed, well-positioned, infection is excluded and both gaps can be corrected without overstuffing.

Operative Technique

Revision for instability: PIPADRAW sequence

Revision Instability Operation

Step	What The Surgeon Does	Pitfall
Position	Supine, tourniquet available, previous incision planned, full limb accessible for alignment and stress testing.	Poor exposure makes component removal and gap assessment unreliable.
Imaging and equipment	Have image intensifier if needed, extraction tools, augments, stems, cones/sleeves, PS, VVC/CCK and hinge options available.	Starting without the next constraint level can force a compromised reconstruction.
Preparation	Antibiotics after cultures if infection concern, extensile exposure plan, aspiration/cultures as indicated.	Do not ignore infection because the referral says instability.
Approach	Use prior midline incision where safe; medial parapatellar arthrotomy is common; plan quadriceps snip or tibial tubercle osteotomy if exposure is difficult.	Avulsing the extensor mechanism changes the entire reconstruction.
Dissection	Assess polyethylene wear, PCL status if CR, collateral integrity, extensor mechanism, component fixation and bone loss.	Do not remove well-fixed components before confirming why the knee is unstable.
Reconstruction	Remove failed components when needed; rebuild tibial platform and femoral joint line; restore posterior offset and correct slope/rotation.	A thicker insert cannot correct a malpositioned femoral or tibial component.
At-risk structures	Protect patellar tendon, MCL, LCL, popliteus, posterior capsule, popliteal vessels and peroneal nerve in valgus knees.	Peroneal nerve risk rises with valgus correction and flexion contracture correction.
Fixation	Use stems for load sharing when constraint, bone loss or augments increase interface stress; use augments/cones/sleeves for bone defects.	Constraint without fixation support increases loosening risk.
Closure and aftercare	Confirm tracking, stability at 0°, 30°, 60° and 90°, ROM, patella tracking and neurovascular status; brace if soft-tissue concern.	Failure to document the final stability arc leaves uncertainty after revision.

CR TKA instability converted to posterior-stabilized TKA — Open-access radiographs showing instability after cruciate-retaining TKA and conversion to a posterior-stabilized construct. This is useful when PCL failure is the dominant problem and collaterals/component position remain manageable.Credit: Chang MJ et al. via Knee Surgery and Related Research / Open-i (CC BY)

Complications and Failure Management

Why Instability Revisions Fail

Failure Mode	Cause	Prevention or Salvage
Persistent flexion instability	Posterior offset not restored, excessive slope persists, PCL failure not addressed.	Revise femur/tibia as needed; PS or VVC depending collateral competence.
New stiffness	Overstuffed knee, excessive insert thickness, joint-line error.	Correct gap cause rather than indiscriminate insert upsizing.
Persistent coronal laxity	Collateral incompetence underestimated.	Increase to VVC/CCK or hinge; reconstruct bone loss and alignment.
Aseptic loosening	High constraint without adequate fixation or bone support.	Use stems, augments, cones/sleeves and appropriate fixation strategy.
Missed infection	Pain and instability treated as mechanical only.	PJI workup before revision and cultures at surgery when indicated.
Extensor mechanism failure	Exposure injury, chronic patellar maltracking or unrecognised extensor deficit.	Plan extensile exposure carefully; reconstruct extensor mechanism when needed.

Evidence Signals

Gap balancing versus measured resection

Review

Key Findings:

The goal of TKA is symmetric balanced flexion and extension gaps.
Gap balancing aims to tension the collaterals and make the femoral component parallel to the tibial cut in flexion.
The review argues gap balancing can produce more rectangular flexion gaps, while acknowledging controversy.

Clinical Implication: Use gap-balancing principles to understand instability even if the surgeon uses a measured-resection or hybrid technique.

Limitation: Narrative review with technique preference; not definitive comparative survivorship evidence.

Source: Daines and Dennis, Clinics in Orthopedic Surgery, 2014

Gap balancing meta-analysis

Meta-analysis

Key Findings:

Eight observational comparison studies were included.
Gap balancing and measured resection had broadly similar soft-tissue balancing outcomes.
Gap balancing showed small differences in femoral external rotation and joint-line change.

Clinical Implication: Do not present gap balancing and measured resection as mutually exclusive dogma; modern TKA often uses hybrid judgement.

Limitation: Included studies were observational with heterogeneity.

Source: Moon et al., Medicine, 2016

Clinical Scenarios

Use these scenarios to practise clinical reasoning and management decisions

CLINICAL SCENARIOStandard

CLINICAL PROMPT

"A patient has recurrent effusions and giving way when descending stairs 18 months after a cruciate-retaining TKA. The knee is stable in extension but has increased anterior-posterior translation at 90°."

PRACTICAL APPROACH

This pattern is most consistent with flexion instability. I would exclude infection, obtain standing AP/lateral/skyline and long-leg films, assess tibial slope, posterior offset, component fixation and rotation, and examine coronal stability at 0°, 30° and 90°. Non-operative care may help mild symptoms, but objective flexion laxity with recurrent effusion usually needs revision. The revision plan is to correct the cause: restore posterior condylar offset, reduce excessive slope, correct malrotation and convert from CR to PS if isolated PCL failure is the driver. If collateral laxity remains, I would increase constraint to VVC/CCK rather than relying on PS alone.

KEY CLINICAL POINTS

Flexion instability is flexion gap greater than extension gap.

Typical symptoms include stairs, chair rise, effusions and subjective instability.

Revision must restore posterior offset and correct slope/rotation.

COMMON PITFALLS

✗Treating with thicker polyethylene alone when extension gap is already balanced.

✗Not excluding infection.

✗Converting to PS despite collateral incompetence.

FURTHER QUESTIONS

"What radiographic measurements are important?"

"When would you choose VVC instead of PS?"

"How does posterior tibial slope affect a CR TKA?"

CLINICAL SCENARIOAdvanced

CLINICAL PROMPT

"During primary TKA for a severe valgus knee, the extension gap remains laterally tight after bone cuts and osteophyte removal. The medial side is attenuated."

PRACTICAL APPROACH

I would first confirm the tibial and distal femoral cuts are correct and remove lateral osteophytes. If the lateral side is tight in extension, the IT band and posterolateral capsule are likely contributors. I would release stepwise and reassess at 0°, 30° and 90° after each release. If flexion is also tight laterally, popliteus or LCL tightness may need to be addressed cautiously. Because the medial side is attenuated, I would not over-release the lateral side expecting a standard PS implant to control the knee. If medial competence is inadequate after balancing, I would choose VVC/CCK or hinge depending residual stability, bone loss and extensor mechanism.

KEY CLINICAL POINTS

Release pattern depends on flexion angle of tightness.

Valgus knees need peroneal nerve and MCL competence awareness.

Constraint is selected when soft tissues cannot stabilise the corrected knee.

COMMON PITFALLS

✗Aggressive lateral release creating a globally unstable knee.

✗Ignoring incompetent MCL.

✗Not testing gaps at multiple flexion angles.

FURTHER QUESTIONS

"Which lateral structures are tight in extension versus flexion?"

"When is a hinge required?"

"What is the peroneal nerve risk in valgus correction?"

References

Daines BK, Dennis DA. Gap balancing vs. measured resection technique in total knee arthroplasty. Clin Orthop Surg. 2014;6(1):1-8. doi:10.4055/cios.2014.6.1.1.
Moon YW, Kim HJ, Ahn HS, Park CD, Lee DH. Comparison of soft tissue balancing, femoral component rotation, and joint line change between the gap balancing and measured resection techniques in primary total knee arthroplasty: a meta-analysis. Medicine. 2016;95(39):e5006. doi:10.1097/MD.0000000000005006.
Siddiqi A, Smith T, McPhilemy JJ, Ranawat AS, Sculco PK, Chen AF. Soft-Tissue Balancing Technology for Total Knee Arthroplasty. JBJS Rev. 2020;8(1):e0050. doi:10.2106/JBJS.RVW.19.00050.
Stambough JB, Edwards PK, Mannen EM, Barnes CL, Mears SC. Flexion Instability After Total Knee Arthroplasty. J Am Acad Orthop Surg. 2019;27(17):642-651. doi:10.5435/JAAOS-D-18-00347.
Cottino U, Sculco PK, Sierra RJ, Abdel MP. Instability After Total Knee Arthroplasty. Orthop Clin North Am. 2016;47(2):311-316. doi:10.1016/j.ocl.2015.09.007.
Chang MJ, Lim H, Lee NR, Moon YW. Diagnosis, causes and treatments of instability following total knee arthroplasty. Knee Surg Relat Res. 2014;26(2):61-67. doi:10.5792/ksrr.2014.26.2.61.
Mehta N, Burnett RA, Kahlenberg CA, Miller R, Chalmers B, Cross MB. Mid-Flexion Instability After Total Knee Arthroplasty: Diagnosis, Implant Design, and Outcomes. Orthopedics. 2023;46(1):e13-e19. doi:10.3928/01477447-20220719-01.
Vajapey SP, Pettit RJ, Li M, Chen AF, Spitzer AI, Glassman AH. Risk Factors for Mid-Flexion Instability After Total Knee Arthroplasty: A Systematic Review. J Arthroplasty. 2020;35(10):3046-3054. doi:10.1016/j.arth.2020.05.026.
Rossi R, Cottino U, Bruzzone M, Dettoni F, Bonasia DE, Rosso F. Total knee arthroplasty in the varus knee: tips and tricks. Int Orthop. 2019;43(1):151-158. doi:10.1007/s00264-018-4116-3.
Alesi D, Meena A, Fratini S, et al. Total knee arthroplasty in valgus knee deformity: is it still a challenge in 2021? Musculoskelet Surg. 2022;106(1):1-8. doi:10.1007/s12306-021-00695-x.
Rezaei A, Moon J, Lichtig A, et al. Precision soft tissue balancing: grid-assisted pie-crusting in total knee arthroplasty. Front Surg. 2024;11:1331902. doi:10.3389/fsurg.2024.1331902.

TKA Balancing and Instability Cheat Sheet

Clinical summary

Examine

•Gait: thrust or recurvatum
•Varus-valgus at 0°, 30°, 90°
•AP drawer at 90°
•Mid-flexion stress
•Extensor mechanism

Image

•Standing AP/lateral/skyline
•Long-leg alignment
•Compare prior films
•CT for rotation
•ESR/CRP and aspirate if suspicious

Correct

•Flexion: posterior offset/slope/PCL
•Extension: distal femoral joint line
•Asymmetric: release or recut
•Global: constraint after reconstruction
•Never miss infection

"Diagnose which gap is wrong, correct the cause, then choose the least constraint that controls the knee."

Finding

Likely Problem

Correction Principle

Loose at 90° but stable in extension

Flexion gap too large.

Increase posterior femoral offset, reduce excessive slope, correct PCL strategy, revise malrotation if present.

Loose in extension but acceptable at 90°

Extension gap too large.

Restore distal femoral joint line with distal augments; do not rely on thicker insert alone.

Loose at 30° to 60° but stable at endpoints

Mid-flexion instability.

Assess joint line, femoral component design/position, collateral isometry and coronal laxity.

Varus or valgus opening at multiple flexion angles

Collateral incompetence, asymmetric release, malalignment or bone loss.

Correct alignment/bone loss and use VVC/CCK or hinge if soft tissues cannot stabilise the knee.

Painful instability with raised inflammatory markers

Possible periprosthetic joint infection.

Investigate infection before any instability revision.

D-P-SWhat changes gaps

PJIBefore revision

CPVHConstraint ladder

Distal femur

Mainly changes extension gap.

Pain pattern

Pain may be infection, loosening, malrotation, instability or extensor failure.

PCL and collaterals competent.

Posterior condyles

Mainly changes flexion gap and posterior offset.

Joint aspiration

Use when infection suspicion persists after blood tests.

PCL deficient but collaterals competent.

Slope

More posterior tibial slope opens flexion gap, especially in CR knees.

Implant position

Do not revise instability without assessing component position.

VVC/CCK

Coronal laxity with usable extensor mechanism.

Hinge

Global instability or severe bone/ligament loss.

Distal, posterior, slope.

Painful loose TKA: infection first.

Constraint rises only when soft-tissue control falls.

Term

Meaning

Why It Matters

Extension gap

Space and soft-tissue tension with the knee at full extension.

Affected mainly by distal femoral resection, posterior capsule, collateral tension and osteophytes.

Flexion gap

Space and soft-tissue tension at 90° flexion.

Affected mainly by posterior condylar resection, femoral sizing, posterior offset, tibial slope and PCL status.

Rectangular gap

Medial and lateral compartments open evenly under tension.

Needed for coronal stability and central tracking.

Asymmetric gap

One compartment remains tighter or looser than the other.

Requires selective release, recut, component correction or constraint depending cause.

Constraint

Mechanical stability built into the implant.

Useful when soft tissues cannot stabilise the knee, but it increases load transfer to fixation interfaces.

Structure

Main Role

Operative Relevance

Superficial MCL

Primary medial restraint, especially against valgus stress.

Over-release in varus TKA causes medial instability and may force higher constraint.

Deep MCL and posteromedial capsule

Medial tightness near the joint line.

Often released early after osteophyte removal in varus knees.

PCL

Posterior stabiliser and flexion-gap restraint in CR TKA.

Incompetence creates flexion instability; excessive tension can limit flexion or lift off.

LCL and posterolateral structures

Lateral stability, especially in valgus knees.

Release sequence depends on whether tightness is in extension, flexion or both.

IT band

Lateral extension tightness.

Release if valgus knee is tight mainly in extension.

Popliteus

Posterolateral flexion tightness.

Release cautiously if valgus knee is tight mainly in flexion.

Posterior osteophytes and capsule

Block extension and distort gap assessment.

Remove osteophytes before judging true extension balance.

Extensor mechanism

Patellar tracking and active extension.

Extensor failure can mimic instability and changes reconstruction choices.

Mechanism

What Happens Biomechanically

Clinical Pattern

Posterior condylar offset loss

Flexion gap opens because the femur is effectively undersized posteriorly.

Giving way in flexion, stairs difficulty and AP drawer at 90°.

Excess posterior tibial slope

Tibia rests in a posteriorly sloped platform, increasing flexion laxity.

Flexion instability, especially in CR TKA with PCL insufficiency.

Excess distal femoral resection

Extension gap opens more than flexion gap.

Recurvatum tendency or instability in stance.

Femoral or tibial malrotation

Flexion gap becomes asymmetric and patellofemoral tracking may fail.

Pain, stiffness, patellar symptoms or asymmetric flexion instability.

Collateral incompetence

The knee cannot resist varus-valgus load despite acceptable bone cuts.

Coronal opening at 0°, 30° or 90°; may require VVC/CCK or hinge.

Joint-line elevation

Collateral isometry and patellofemoral mechanics change through mid-flexion.

Possible mid-flexion instability, stiffness or extensor symptoms.

Step

Action

Decision Point

Remove medial tibial and femoral osteophytes.

If this corrects the gap, stop releasing.

Release deep MCL and posteromedial capsule from proximal tibia.

Good first soft-tissue step for most fixed varus knees.

Release semimembranosus if posteromedial tightness persists.

Useful when extension and posteromedial flexion remain tight.

Pie-crust or cautiously lengthen superficial MCL if severe medial tightness remains.

Risk is medial instability; reassess repeatedly.

Consider reduction osteotomy, sliding medial epicondyle osteotomy or constraint for severe fixed deformity.

Do this instead of uncontrolled MCL destruction.

Tightness Pattern

Likely Structure

Surgical Response

Tight mainly in extension

IT band and posterolateral capsule.

Release extension-side lateral structures first and reassess.

Tight mainly in flexion

Popliteus or LCL depending pattern.

Release flexion-side structure cautiously; avoid creating lateral flexion laxity.

Tight in flexion and extension

LCL and broader posterolateral complex.

Sequential release; high chance of needing increased constraint.

Medial side incompetent

Attenuated MCL in severe valgus.

Do not rely on soft-tissue release alone; consider VVC/CCK or hinge.

Test

How To Perform It

Positive Meaning

Gait observation

Watch walking from front, side and back; look for varus/valgus thrust, recurvatum, limp and extensor lag.

Thrust suggests coronal instability or malalignment; recurvatum suggests extension instability or neuromuscular control problem.

Varus-valgus stress at 0°

Patient supine, knee fully extended, stabilise femur and apply varus/valgus force at ankle.

Opening in extension suggests collateral incompetence, excessive extension gap or global instability.

Varus-valgus stress at 30°

Unlock the knee slightly and repeat stress testing.

Isolates collateral behaviour better than full extension; useful for mid-flexion/coronal laxity.

Varus-valgus stress at 90°

Knee flexed to 90°, foot supported, stress through the tibia.

Opening in flexion suggests flexion-gap asymmetry or collateral incompetence in flexion.

AP drawer at 90°

Stabilise foot, flex knee to 90°, translate tibia anterior and posterior.

Excess AP translation suggests flexion instability, PCL failure in CR TKA or post-cam problems in PS TKA.

Mid-flexion stress

Test coronal stability around 30° to 60° with the patient relaxed.

Instability here with stable endpoints supports mid-flexion instability.

Extensor mechanism

Assess active straight-leg raise, extensor lag, patellar tracking and apprehension.

Extensor failure or patellar instability can mimic giving way and changes reconstruction plan.

Measurement

How To Assess

Why It Changes Treatment

Mechanical alignment

Full-length standing film: femoral head centre to ankle centre through knee.

Malalignment may drive coronal thrust and demands component or osteotomy-level correction.

Component coronal position

AP knee and long-leg films.

Varus tibial or valgus/varus femoral error can create asymmetric gaps.

Tibial posterior slope

True lateral radiograph.

Excess slope increases flexion gap, especially in CR TKA.

Posterior condylar offset

Lateral radiograph and comparison to pre-op/early post-op if available.

Loss of offset opens flexion gap and can cause flexion instability.

Joint-line elevation

Compare with fibular head, adductor tubercle or prior films.

Elevation can contribute to mid-flexion instability, patellar problems and extensor dysfunction.

Component rotation

CT using recognised femoral and tibial rotational references.

Malrotation can cause flexion-gap asymmetry, patellar maltracking, stiffness and pain.

Fixation and bone loss

Radiolucent lines, migration, osteolysis, CT if needed.

Loose or deficient components need revision reconstruction, not soft-tissue-only treatment.

Situation

Treatment

Limit

Mild subjective symptoms, stable implant, no progressive laxity

Physiotherapy for quadriceps/hip strength, gait retraining and proprioception.

Does not correct mechanical gap mismatch.

Low-demand patient with mild coronal laxity and no loosening

Functional brace and activity modification.

Temporary or palliative option; monitor progression.

Early flexion symptoms without objective gross laxity

Strengthening, swelling control and careful review.

Persistent recurrent effusion or objective drawer needs surgical planning.

Infection suspected

Do not start definitive instability treatment until infection pathway is complete.

Missed infection makes revision fail.

Step

What The Surgeon Does

Pitfall

Position

Supine, tourniquet available, previous incision planned, full limb accessible for alignment and stress testing.

Poor exposure makes component removal and gap assessment unreliable.

Imaging and equipment

Have image intensifier if needed, extraction tools, augments, stems, cones/sleeves, PS, VVC/CCK and hinge options available.

Starting without the next constraint level can force a compromised reconstruction.

Preparation

Antibiotics after cultures if infection concern, extensile exposure plan, aspiration/cultures as indicated.

Do not ignore infection because the referral says instability.

Approach

Use prior midline incision where safe; medial parapatellar arthrotomy is common; plan quadriceps snip or tibial tubercle osteotomy if exposure is difficult.

Avulsing the extensor mechanism changes the entire reconstruction.

Dissection

Assess polyethylene wear, PCL status if CR, collateral integrity, extensor mechanism, component fixation and bone loss.

Do not remove well-fixed components before confirming why the knee is unstable.

Reconstruction

Remove failed components when needed; rebuild tibial platform and femoral joint line; restore posterior offset and correct slope/rotation.

A thicker insert cannot correct a malpositioned femoral or tibial component.

At-risk structures

Protect patellar tendon, MCL, LCL, popliteus, posterior capsule, popliteal vessels and peroneal nerve in valgus knees.

Peroneal nerve risk rises with valgus correction and flexion contracture correction.

Fixation

Use stems for load sharing when constraint, bone loss or augments increase interface stress; use augments/cones/sleeves for bone defects.

Constraint without fixation support increases loosening risk.

Closure and aftercare

Confirm tracking, stability at 0°, 30°, 60° and 90°, ROM, patella tracking and neurovascular status; brace if soft-tissue concern.

Failure to document the final stability arc leaves uncertainty after revision.

Failure Mode

Cause

Prevention or Salvage

Persistent flexion instability

Posterior offset not restored, excessive slope persists, PCL failure not addressed.

Revise femur/tibia as needed; PS or VVC depending collateral competence.

New stiffness

Overstuffed knee, excessive insert thickness, joint-line error.

Correct gap cause rather than indiscriminate insert upsizing.

Persistent coronal laxity

Collateral incompetence underestimated.

Increase to VVC/CCK or hinge; reconstruct bone loss and alignment.

Aseptic loosening

High constraint without adequate fixation or bone support.

Use stems, augments, cones/sleeves and appropriate fixation strategy.

Missed infection

Pain and instability treated as mechanical only.

PJI workup before revision and cultures at surgery when indicated.

Extensor mechanism failure

Exposure injury, chronic patellar maltracking or unrecognised extensor deficit.

Plan extensile exposure carefully; reconstruct extensor mechanism when needed.

Gap balancing versus measured resection

Review

Key Findings:

The goal of TKA is symmetric balanced flexion and extension gaps.
Gap balancing aims to tension the collaterals and make the femoral component parallel to the tibial cut in flexion.
The review argues gap balancing can produce more rectangular flexion gaps, while acknowledging controversy.

Clinical Implication: Use gap-balancing principles to understand instability even if the surgeon uses a measured-resection or hybrid technique.

Limitation: Narrative review with technique preference; not definitive comparative survivorship evidence.

Source: Daines and Dennis, Clinics in Orthopedic Surgery, 2014

Gap balancing meta-analysis

Meta-analysis

Key Findings:

Eight observational comparison studies were included.
Gap balancing and measured resection had broadly similar soft-tissue balancing outcomes.
Gap balancing showed small differences in femoral external rotation and joint-line change.

Clinical Implication: Do not present gap balancing and measured resection as mutually exclusive dogma; modern TKA often uses hybrid judgement.

Limitation: Included studies were observational with heterogeneity.

Source: Moon et al., Medicine, 2016

Failure Pattern	Do Not Do	Preferred Strategy
Flexion instability	Do not simply thicken insert if extension gap is already balanced.	Increase posterior offset, reduce excessive slope, correct rotation, convert CR to PS/VVC depending collateral competence.
Extension instability	Do not overstuff flexion gap with insert alone.	Restore distal femoral joint line with augments, revise malposition and increase constraint if collateral deficient.
Mid-flexion instability	Do not diagnose only at 0° and 90°.	Assess 30° to 60°, joint line, femoral design, component position, collateral isometry and malrotation.
Coronal instability	Do not leave malalignment uncorrected and rely on constraint alone.	Revise alignment, reconstruct bone loss and use VVC/CCK if collateral support remains incomplete.
Global instability	Do not use PS when collaterals cannot control the knee.	Rotating hinge if VVC cannot control instability or bone/ligament loss is severe.

Failure Pattern	Do Not Do	Preferred Strategy
Flexion instability	Do not simply thicken insert if extension gap is already balanced.	Increase posterior offset, reduce excessive slope, correct rotation, convert CR to PS/VVC depending collateral competence.
Extension instability	Do not overstuff flexion gap with insert alone.	Restore distal femoral joint line with augments, revise malposition and increase constraint if collateral deficient.
Mid-flexion instability	Do not diagnose only at 0° and 90°.	Assess 30° to 60°, joint line, femoral design, component position, collateral isometry and malrotation.
Coronal instability	Do not leave malalignment uncorrected and rely on constraint alone.	Revise alignment, reconstruct bone loss and use VVC/CCK if collateral support remains incomplete.
Global instability	Do not use PS when collaterals cannot control the knee.	Rotating hinge if VVC cannot control instability or bone/ligament loss is severe.