High-yield overview
Fracture | Instability | Clunk Syndrome | Aseptic Loosening
1-50%Incidence (Historical)
0.5-1%Fracture Rate
GoldbergFracture Classification
UrgentExtensor Lag
GOLDBERG CLASSIFICATION (FRACTURES)
Type I
PatternExtensor mechanism INTACT + Implant STABLE
TreatmentNon-operative
Type II
PatternExtensor mechanism INTACT + Implant LOOSE
TreatmentRevision (Button/Component)
Type III
PatternExtensor mechanism DISRUPTED + Implant STABLE
TreatmentORIF/Repair
Type IV
PatternExtensor mechanism DISRUPTED + Implant LOOSE
TreatmentRevision + Repair
Critical Must-Knows
- Patellar complications are the most common cause of TKA reoperation (historically up to 50%)
- Extensor mechanism integrity is the key determinant of management for fractures
- Patellar clunk syndrome presents as a painful 'clunk' at 30-45 degrees of flexion (extension from flexion)
- Malrotation of femoral/tibial components is a major cause of patellar instability
- Avascular necrosis risk is increased by lateral retinacular release (sacrifices superior lateral genicular artery)
Clinical Pearls
- “Look for component malrotation on axial CT (Berger Protocol)
- “Patellar clunk is treated with arthroscopic debridement of the fibrous nodule
- “Goldberg classification drives treatment: Is the extensor mechanism working? Is the implant loose?
- “Avoid ORIF for patellar fractures in TKA if possible - high non-union rate (poor bone stock)
Clinical Imaging
Imaging Atlas
Critical TKA Patellar Exam Points
Extensor Integrity
Must assess active straight leg raise. Inability implies Type III/IV fracture or tendon rupture requiring surgery.
Component Malposition
Internal rotation of femoral or tibial component is a classic cause of instability and anterior knee pain.
Blood Supply
Superior Lateral Genicular Artery. At risk during lateral release. Damage leads to patellar AVN and fracture.
Patellar Clunk
Fibrous nodule at superior pole. Clunks at 30-45° flexion. Differentiate from crepitus or instability.
| Scenario | Diagnosis | Treatment | Key Pearl |
|---|---|---|---|
| Painful clunk at 30-45° flexion | Patellar Clunk Syndrome | Arthroscopic Debridement | Resect fibrous nodule |
| Fracture, SLR possible, Implant stable | Goldberg Type I | Non-operative (Brace) | Watch for displacement |
| Fracture, SLR possible, Implant LOOSE | Goldberg Type II | Revision Arthroplasty | Remove loose button/cement |
| Fracture, NO SLR, Implant stable | Goldberg Type III | Surgical Repair/ORIF | High failure rate |
| Patellar dislocation, component malrotation | Patellar Instability | Revision of Malrotated Component | Soft tissue release fails if malrotated |
Mnemonic
MALROTATIONCauses of Patellar Instability
M
Malrotation
Internal rotation of femur/tibia (Most Common)
A
Alignment
Valgus limb alignment
L
Lateralization
Lateralized patellar component
R
Retinaculum
Tight lateral retinaculum
O
Overstuffing
Thick patellar composite/anterior femur
T
Trauma
Traumatic disruption of MPFL
Hook:MALROTATION involves Malposition, Alignment, Lateralization - Remember internal rotation is the enemy!
Mnemonic
MILSGoldberg Classification (Fractures)
M
Mechanism
Extensor Mechanism Status (Intact vs Disrupted)
I
Implant
Implant Stability (Stable vs Loose)
L
Location
Fracture location relative to implant
S
Stock
Bone stock quality
Hook:MILS helps decide if surgery is needed - Mechanism and Implant stability are key.
Mnemonic
SAVERisks of Lateral Release
S
Superior
Superior Lateral Genicular Artery is at risk
A
Avascular
Risk of Avascular Necrosis (AVN)
V
Vessel
Vessel runs in the substance of the retinaculum
E
Edge
Leave a cuff of tissue if possible
Hook:SAVE the Superior Lateral Genicular Artery to prevent AVN and fracture!
Overview and Epidemiology
Historical Context
Historically, patellar complications were responsible for up to 50% of TKA failures. Modern implant designs (smoother trochlea, better instrumentation) have significantly reduced this, but it remains a leading cause of reoperation.
Risk Factors: Fracture
- Vascular: Lateral release (AVN)
- Technical: Over-resection (less than 12mm), excessive thermal necrosis
- Patient: Osteoporosis, high activity, male gender
Risk Factors: Instability
- Component: Internal rotation (Femur/Tibia), Medialization of femoral component
- Soft Tissue: Tight lateral retinaculum, loose MCL (valgus)
Anatomy and Biomechanics
Blood Supply: The Critical Concept
The patella is supplied primarily by the genicular anastomosis. The Superior Lateral Genicular Artery and Inferior Lateral Genicular Artery are most critical. A wide lateral retinacular release sacrifices the Superior Lateral Genicular Artery, significantly increasing the risk of patellar AVN and subsequent fragmentation/fracture.
| Variable | Effect on Patella | Surgical Pearl |
|---|---|---|
| Internal Rotation (Femur) | Increases Q-angle | Avoid internal rotation! Use Whiteside's line/TEA |
| Internal Rotation (Tibia) | Lateralizes Tubercle | Align to medial 1/3 tibial tubercle |
| Medialized Femoral Comp | Increases Q-angle | Lateralize femoral component to improve tracking |
| Thick Patella (Overstuffing) | Increases shear force | Measure pre-op thickness and reproduce it |
Classification Systems
Goldberg Classification
The standard classification for periprosthetic patellar fractures.
| Type | Mechanism Intact? | Implant Stable? | Treatment |
|---|---|---|---|
| Type I | YES | YES | Non-operative (Imm. Range of Motion) |
| Type II | YES | NO | Revision (Excise loose button/cement) |
| Type III | NO (Disrupted) | YES | Surgery (Repair/ORIF + liner exchange) |
| Type IV | NO (Disrupted) | NO | Surgery (Resection/Patellectomy/Allograft) |
Type II Pearl
For Type II (Loose implant, Intact mechanism), if the bone stock is poor, it is often better to remove the loose button and leave the patella unresurfaced (patelloplasty) rather than attempting to recement a new button onto a shell of bone.
Clinical Assessment
History
- Instability: "Knee gives way", "Patella jumps out"
- Clunk: Painful catch/clunk at 30-45 degrees flexion
- Fracture: Acute pain, inability to straight leg raise
- Pain: Anterior knee pain, worse with stairs/rising
Examination
- Observation: Q-angle, Valgus alignment
- Palpation: Fibrous nodule (Clunk), Tenderness
- Dynamic: Patellar tracking (J-sign), Active SLR (Mechanism integrity)
- Implant: Assess for gross loosening/instability
Rule Out Infection
In any painful TKA, infection must be ruled out first. Aseptic loosening and patellar complications can mimic infection. Obtain ESR/CRP and aspirate if elevated.
Investigations
Imaging Protocol
X-raysStandard Series
- AP/Lateral: Fracture, implant position, bone stock
- Skyline (Merchant): Critical for assessment of patellar tilt, subluxation, and fracture profile
CT ScanRotational Profile (Berger)
- Indication: Instability or Anterior Knee Pain
- Assess: Rotational alignment of femoral and tibial components
- Femur: PCA (Posterior Condylar Axis) vs TEA (Transepicondylar Axis)
- Tibia: Geometric center vs Tubercle
Bone ScanOccult Loosening
- Indication: Pain with normal X-rays/CT
- Finding: Increased uptake ('hot') patella may suggest stress fracture or loosening
Berger Protocol
The Berger Protocol typically combines femoral and tibial rotation.
- Internal Rotation of Femoral Comp + Internal Rotation of Tibial Comp = Severe Patellar Instability. Even minor internal rotation of both can summate to cause significant tracking issues.
Management Algorithm
Management of Patellar Fracture
Based on Goldberg Classification.
Principles:
- Preserve extensor mechanism (Priority #1)
- Preserve bone stock
- Avoid surgery if mechanism is intact (Type I)
Treatment Steps
ObservationType I
- Cylinder cast/brace in extension for comfort
- Allow weight bearing as tolerated
- Start immediate ROM (if stable) to prevent stiffness
SurgeryType II
- Extensor mechanism is intact but button is loose.
- Surgery: Remove loose button.
- Decision: Recement (if good bone greater than 12mm) vs Resection Arthroplasty (Patelloplasty).
SurgeryType III
- Extensor disrupted, Button stable.
- Surgery: Repair extensor mechanism + ORIF (Tension band).
- Outcome: Poor. High failure rate. Consider allograft if repair poor.
ComplexType IV
- Disrupted + Loose.
- Surgery: Remove button + Reconstruct mechanism (Allograft/Mesh).
- Typically requires removal of loose bodies and major reconstruction.
Management decisions depend on patient factors and fracture stability.
Surgical Technique
Revision for Patellar Instability (Malrotation)
Surgical Steps
Step 1Exposure
- Medial parapatellar approach (standard).
- Extensive synovectomy.
- Inspect patellar tracking before dislocating/everting.
Step 2Assess Rotation
- Use epicondylar axis (TEA) and Whiteside's line.
- Confirm internal rotation of femoral component.
Step 3Component Removal
- Remove femoral component carefully (preserve bone).
- Osteotomes/Gigli saw if cemented.
Step 4Re-implantation
- External Rotation: Set femoral component in 3 degrees external rotation relative to posterior condyles (or parallel to TEA).
- Lateralization: Use an offset stem or shift component laterally to improve tracking.
Step 5Verification
- "No Thumbs" test: Patella should track centrally without thumb pressure throughout ROM.
Complications
| Complication | Risk | Management | Prevention |
|---|---|---|---|
| Extensor Mechanism Rupture | High in Type III/IV | Allograft/Mesh Reconstruction | Avoid aggressive ROM |
| Avascular Necrosis | Lateral Release | Observation over Patellectomy | Preserve Sup Lat Genicular Artery |
| Non-union of Fracture | Common (Poor bone) | Accept if painless/stable | Optimized fixation |
| Recurrent Instability | Malrotation not addressed | Revision TKA | Correct component rotation |
Postoperative Care
Rehabilitation Protocol
0-2 WeeksImmobilization
- Fracture/Repair: Hinged knee brace locked in extension.
- Weight Bearing: WBAT with brace locked in extension.
- Revision TKA: Often WBAT, start ROM immediately if stable.
2-6 WeeksProtected ROM
- Fracture: Start passive ROM limited to 0-30 degrees (if repair strong).
- Clunk Resection: Immediate full ROM allowed.
- X-rays: Check at 2 weeks and 6 weeks for alignment/displacement.
6-12 WeeksMobilization
- Unlock brace for walking (if SLR control good).
- Progressive flexion as tolerated.
- Strengthening (quadriceps) starts late (3 months) for fractures.
3-6 MonthsReturn to Function
- Full functional activity.
- Monitor for late loosening or recurrence of instability.
Strict adherence to range of motion limits is required for extensor repairs.
Outcomes and Prognosis
Patellar Fracture
- High Complication Rate: Surgical repair fails in 20-30% of cases.
- Non-union: Common (30-50%) but often painless if Type I.
- Best Outcome: Type I treated non-operatively (96% success).
Patellar Clunk
- Excellent: Arthroscopic debridement has greater than 95% success rate.
- Recurrence: Rare if sufficient nodule resected.
Instability
- Revision: Success depends on identifying cause.
- Malrotation: Revising components has 80-90% success.
- Soft Tissue Only: Lateral release alone has high failure if malrotation missed.
Extensor Rupture
- Poor Prognosis: Primary repair has high failure rate.
- Reconstruction: Allograft/Mesh often required.
- Salvage: Arthrodesis may be end stage.
Evidence Base
Evidence
Goldberg Classification of Periprosthetic Patellar Fracture
Goldberg VM, Figgie HE, et al. • Clin Orthop Relat Res (1988)
Key Findings:
- Established the widely used types of periprosthetic patellar fracture
- Key determinants: extensor mechanism integrity and implant stability
- Type I (intact mechanism, stable implant) has the best prognosis
- Disrupted-mechanism and loose-implant types have poor outcomes
- Note: a classic classification paper; no verifiable PubMed record located
Clinical implication: Classify fracture by fracture location, extensor mechanism integrity and implant stability to determine operative vs non-operative management.
Evidence
The Patellar Clunk Syndrome
Hozack WJ, Rothman RH, Booth RE, Balderston RA • Clin Orthop Relat Res (1989)
Key Findings:
- Original description in three patients after posterior-stabilised TKA
- Painful 'catch' or 'clunk' on knee extension
- Prominent fibrous nodule at junction of proximal patellar pole and quadriceps tendon
- Surgical removal of the nodule resolved symptoms
Clinical implication: Suspect a suprapatellar fibrous nodule in posterior-stabilised knees presenting with a painful catch on extension; nodule excision resolves symptoms.
Evidence
Periprosthetic Patellar Fracture: Classification & Outcomes
Ortiguera CJ, Berry DJ • J Bone Joint Surg Am (2002)
Key Findings:
- 85 fractures (77 patients) after 12,464 TKAs - overall prevalence 0.68%
- Significantly more common in men (1.01%) than women (0.40%)
- Type I (intact mechanism + stable implant, n=38): nonoperative treatment with only one late failure
- Operative treatment (disrupted mechanism or loose component) had high complication and reoperation rates
Clinical implication: Treat by extensor mechanism integrity and implant fixation; avoid surgery when the mechanism is intact and the implant is stable, as operative treatment carries a high complication rate.
Evidence
Malrotation Causing Patellofemoral Complications
Berger RA, Crossett LS, Jacobs JJ, Rubash HE • Clin Orthop Relat Res (1998)
Key Findings:
- 30 patients with patellofemoral complications vs 20 well-functioning controls
- Complication group had excessive combined (femoral + tibial) internal rotation; controls were in external rotation
- Combined internal rotation 1-4 deg: lateral tracking/tilt; 3-8 deg: subluxation; 7-17 deg: dislocation or late prosthesis failure
- Transepicondylar axis and tibial tubercle are reproducible CT landmarks
Clinical implication: Quantify femoral and tibial component rotation on CT before soft-tissue procedures; combined internal rotation predicts the severity of patellofemoral complications and may mandate revision.
Evidence
Lateral Release, Patellar AVN & Stress Fracture
Scott RD, Turoff N, Ewald FC • Clin Orthop Relat Res (1982)
Key Findings:
- Patellar stress fracture incidence 0.7% in rheumatoid vs 3.5% in osteoarthritic knees
- Osteonecrosis occurred in at least three cases where lateral release had sacrificed the superior lateral genicular artery
- Vessel should be preserved to protect patellar and lateral skin flap blood supply
- Preserving peripheral cortex and bone stock reduces fracture risk
Clinical implication: Preserve the superior lateral genicular artery during lateral release and minimise patellar bone resection to reduce osteonecrosis and stress fracture.
Evidence
Patellar Resurfacing vs Non-resurfacing: Meta-analysis
Chen K, Dai X, Li L, et al. • J Orthop Surg Res (2021)
Key Findings:
- 32 randomized trials, 6887 knees
- Resurfacing significantly reduced reoperation (overall and at 5+ years) and noise
- Higher Knee Society and function scores with resurfacing
- No significant difference in anterior knee pain, ROM or patient satisfaction
Clinical implication: Resurfacing reduces reoperation and patellofemoral noise at 5 years or more; weigh against the absence of difference in anterior knee pain and patient satisfaction.
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Scenario 1: Patellar Clunk
Clinical prompt
“A 68-year-old female, 1 year post-op PS-TKA. Doing well but complains of a painful 'catch' or 'clunk' when she stands up from a chair. No pain at rest. ROM 0-120.”
Practical approach
This history is classic for **Patellar Clunk Syndrome**. The key feature is the painful catch during extension from flexion (usually ~30-45 degrees). This is caused by a fibrous nodule at the superior pole of the patella catching in the intercondylar box of the femoral component.
I would examine for a palpable clunk at that range. I would rule out infection and loosening with X-rays (which are likely normal).
Management:
1. **Conservative**: Brief trial, but rarely effective.
2. **Surgical**: Arthroscopic debridement of the nodule is the gold standard. I would counsel her that recurrence is rare after resection.
Key clinical points
Diagnosis: Patellar Clunk Syndrome
Mechanism: Fibrous nodule catching in box
Diagnostic Range: 30-45 degrees flexion
Treatment: Arthroscopic debridement
Common pitfalls
Confusing with arthrofibrosis (stiffness) - Clunk has good ROM
Ordering MRI (not needed, clinical diagnosis + rule out other causes with X-ray)
Further questions
“What design feature increases risk?”
“Does this happen in Cruciate Retaining (CR) knees?”
Viva scenarioChallenging
Scenario 2: The Fracture
Clinical prompt
“A 75-year-old male, 5 years post-TK. Falls onto knee. Pain, swelling. X-ray shows a transverse fracture of the patella. The button appears well-fixed.”
Practical approach
This is a periprosthetic patellar fracture. My immediate priority is to assess the **integrity of the extensor mechanism** and the **stability of the implant**.
On examination, I must ask the patient to perform an **active straight leg raise (SLR)**.
If he CAN perform an SLR (Mechanism Intact) and the implant is stable on X-ray, this is a **Goldberg Type I**.
Management would be non-operative:
1. Knee immobilizer/brace in extension.
2. Weight bearing as tolerated.
3. Serial X-rays to ensure no displacement.
Operative intervention for Type I fractures has a higher complication rate than the fracture itself.
Key clinical points
Critical Exam: Straight Leg Raise (Extensor mechanism)
Classification: Goldberg Type I
Treatment: Non-operative
Risk of surgery: High non-union/necrosis
Common pitfalls
Jumping to surgery for a displaced fracture if SLR is intact
Not assessing the implant stability
Further questions
“What if he CANNOT perform an SLR?”
“What if the button was loose?”
Viva scenarioAdvanced
Scenario 3: Evaluation of Instability
Clinical prompt
“A patient presents with anterior knee pain and feelings of instability 2 years post-TKA. She feels the kneecap 'jumps'. On exam, she has a positive J-sign and lateral apprehension.”
Practical approach
This suggests **Patellar Instability**. I need to determine the cause: is it component malposition or soft tissue imbalance?
My workup includes:
1. **Infection screen** (ESR/CRP) - rule out occult infection.
2. **X-rays**: AP/Lat/Skyline (assess tilt/subluxation).
3. **CT Scan (Rotational Profile)**: This is critical. I need to measure the rotation of the femoral and tibial components (Berger Protocol).
If the CT shows **internal rotation** of the femoral or tibial component, soft tissue procedures (like lateral release) will FAIL. The treatment must be **revision arthroplasty** to correct the rotation.
If rotation is normal, I would consider proximal realignment or MPFL reconstruction.
Key clinical points
Differential: Component vs Soft Tissue
Investigation: CT for Rotational Profile
Rule: Malrotation requires Revision
Pitfall: Lateral release for malrotated component
Common pitfalls
Performing a lateral release without a CT scan
Missing infection as a cause of vague pain
Further questions
“Which artery is at risk during lateral release?”
“What targets do you use for component rotation?”
MCQ Practice Points
Vascular Risk in Lateral Release
Q: Which arterial structure is most at risk during a lateral retinacular release in TKA? A: Superior Lateral Genicular Artery (SLGA) - The SLGA runs in the lateral retinaculum. A wide lateral release can transect this vessel, compromising patellar blood supply and increasing the risk of avascular necrosis and fracture.
Management of Type I Fracture
Q: A patient with a well-fixed TKA sustains a patellar fracture after a fall. She is able to perform a straight leg raise. The implant appears stable on X-ray. What is the Goldberg classification and appropriate management? A: Type I - Non-operative - This is a Goldberg Type I fracture (Intact extensor mechanism + Stable implant). The active straight leg raise confirms mechanism integrity. Treatment is non-operative (immobilizer/brace) as surgical intervention has a high complication rate.
Cause of Late Instability
Q: What is the most likely cause of late-presenting patellar instability after TKA? A: Component Malrotation - While all can cause issues, component malrotation (especially internal rotation of the femoral or tibial component) is a leading cause of mechanical patellar instability. Polyethylene wear can cause late instability but malrotation is the primary mechanical driver.
Extensor Mechanism Rupture Management
Q: Why is primary repair of a chronic patellar tendon rupture in TKA contraindicated? A: High Failure Rate - Primary repair of chronic disruptions (especially in revision scenarios) has an excessively high failure rate. Management requires reconstruction using either an extensor mechanism allograft or synthetic mesh (e.g., Marlex), rather than simple suture repair.
Patella Baja and Joint Line
Q: How does excessive joint line elevation affect the patella? A: Pseudo-Patella Baja - elevating the joint line (e.g., with thick poly or distal femoral augments) causes "pseudo-patella baja". The patella tendon length is normal, but the patella impinges on the tibial post or poly during flexion, reducing ROM and causing anterior pain.
Guidelines, Registries & Global Practice
Global Epidemiology
Periprosthetic patellar fracture is uncommon (around 0.7% of TKAs; Ortiguera & Berry), but patellofemoral problems collectively remain a leading reason for reoperation. Risk rises with resurfacing, lateral release, over-resection (composite under 12 mm), and component malrotation. Reported rates vary widely with implant design and whether the patella was resurfaced.
| Source | Patellar Resurfacing | Key Recommendation |
|---|---|---|
| AAOS / US practice | Selective or routine (surgeon-dependent) | No single mandate; tailor to design, anatomy, anterior knee pain risk |
| BOA / NICE (UK) | Either acceptable | Decision should be informed and design-specific; rule out infection before revision |
| AO Foundation | N/A (fracture focus) | Manage periprosthetic patellar fracture by extensor integrity + implant fixation |
| EFORT / European consensus | Selective | Optimise component rotation; avoid routine lateral release |
| Registries (NJR, AJRR, AOANJRR, NZJR) | Lower reoperation when resurfaced | Resurfacing associated with reduced secondary patellar surgery |
Registry Signals
- Joint registries (NJR UK, AJRR US, AOANJRR Australia, NZJR) consistently show lower rates of secondary patellar surgery when the patella is resurfaced.
- Level I meta-analysis (Chen 2021) concurs: reduced reoperation and noise, no difference in anterior knee pain.
High- vs Limited-Resource Practice
- Rule out PJI first: aspiration plus ESR/CRP before any revision for "pain" or "instability" - a universal standard.
- High-resource: extensor allograft and synthetic mesh available; rotational CT readily obtained.
- Limited-resource: allograft scarce, so autograft/mesh and tertiary referral are emphasised; reliance on skyline radiographs where CT access is limited.
Exam day cheat sheet
TKA PATELLAR COMPLICATIONS
Key Classifications
- Goldberg I: Intact mech, Stable implant → Non-op
- Goldberg II: Intact mech, Loose implant → Revision/Patelloplasty
- Goldberg III: Disrupted mech, Stable implant → Repair + Fixation
- Goldberg IV: Disrupted mech, Loose implant → Salvage/Reconstruct
Patellar Clunk
- Fibrous nodule at superior pole
- Catches in intercondylar box (PS knees)
- Clunk at 30-45° extension
- Rx: Arthroscopic debridement
Instability Causes
- Internal Rotation of Femoral Comp
- Internal Rotation of Tibial Comp
- Valgus alignment
- Tight lateral retinaculum
- Medialized femoral component
Management Pearls
- Avoid surgery for Type I fractures (high complication rate)
- CT scan critical for instability (assess rotation)
- Lateral release endangers Superior Lateral Genicular Artery
- Berger Protocol: Combined internal rotation predicts instability
Blood Supply
- Genicular anastomosis
- Superior Lateral Genicular Artery (Most important)
- Inferior Lateral Genicular Artery
- Avoid deep lateral dissection
Evidence
- Ortiguera & Berry: nonop for intact mechanism/stable implant; surgery has high complications
- Berger: combined internal rotation predicts patellar complication severity (CT)
- Hozack: defined patellar clunk syndrome (PS designs)
- Chen 2021 / registries: resurfacing lowers reoperation rate
- Scott: lateral release compromises SLGA, risks AVN/stress fracture