Infrapatellar Rupture and Repair
- Definition: Disruption of the patellar ligament linking the inferior pole of the patella to the tibial tubercle, abolishing the extensor mechanism
- Demographics: Typically males under 40; quadriceps tendon rupture is the disease of patients over 40
- Mechanism: Sudden eccentric quadriceps contraction against a flexed knee (landing from a jump, missed step) — most commonly at the inferior pole
- Diagnostic triad: palpable infrapatellar gap + loss of active knee extension (no straight-leg raise) + patella alta on lateral radiograph
- Management: Complete ruptures require early surgical repair — never managed conservatively
- “Patella alta with Insall-Salvati greater than 1.2 is the hallmark radiographic sign
- “Ultrasound is a rapid bedside confirmatory test; MRI clarifies level and tissue quality
- “Bilateral or spontaneous rupture = systemic disease until proven otherwise
- “Outcomes are good but full power recovery takes 6-12 months; stiffness is the main complication
Patellar Tendon Rupture
Patella Alta (High riding patella) is the hallmark X-ray sign. Use the Insall-Salvati Ratio (Length of Tendon / Length of Patella). Normal is ~1.0. Greater than 1.2 = Alta. Rupture most commonly occurs at the inferior pole of the patella (Proximal end of tendon).
T-PInsall-Salvati Ratio
Hook:The Tendon is Taller (alTa) when the patellar tendon is ruptured
STRAIGHT LEGSystemic Risk Factors for Tendon Rupture
Hook:A weak tendon cannot hold a STRAIGHT LEG raise
Overview & Epidemiology
Patellar tendon (more correctly patellar ligament) rupture is the least common of the three extensor-mechanism injuries, after patella fracture and quadriceps tendon rupture. It is the third most common extensor-mechanism disruption overall but the dominant one in younger, athletic patients.
Key epidemiological points:
- Predominantly males under 40 years, often athletic. Quadriceps tendon rupture is the corresponding injury in patients over 40.
- Almost always a complete rupture follows pre-existing tendinopathy — the tendon rarely fails when truly healthy.
- Up to roughly one-third of patients have a local risk factor (antecedent tendinopathy, jumper's knee, prior peritendinous corticosteroid injection, prior ACL bone–patellar tendon–bone harvest) or a systemic risk factor; systemic factors dominate in bilateral ruptures.
- Bilateral simultaneous rupture is rare and, when it occurs after low-energy trauma, signifies systemic disease (chronic renal failure, hyperparathyroidism, diabetes, SLE, rheumatoid arthritis, chronic corticosteroid or anabolic-steroid use, fluoroquinolone exposure).
Anatomy & Biomechanics
- A continuation of the central quadriceps tendon, running from the inferior pole of the patella to the tibial tubercle.
- Length approximately 4-6 cm; width approximately 25-30 mm; thickness approximately 5-7 mm.
- Blood supply is from the infrapatellar fat pad posteriorly and the peritendinous retinacular plexus anteriorly. There is a relatively hypovascular zone at the proximal third / inferior pole, which corresponds to the most common rupture site.
- The extensor mechanism transmits very high tensile loads — peak patellofemoral and tendon forces can reach multiples of body weight during jumping and stair descent.
- The patella acts as a fulcrum that increases the moment arm of the quadriceps; loss of patellar tendon continuity abolishes active terminal extension and the ability to lock the knee in stance.
Pathophysiology
- The classic mechanism is sudden eccentric quadriceps contraction against a flexed knee (landing from a jump, missing a step), with the knee usually flexed about 60 degrees at the moment of failure.
- Failure occurs when the applied eccentric load exceeds tendon tensile strength. In a healthy tendon this requires extreme force; in a degenerate tendon, ordinary activity (stumbling, rising from a chair) can suffice.
- Histologically, ruptured tendons typically show mucoid / angiofibroblastic degeneration (tendinosis) rather than acute inflammation, which explains the friable "mop-end" tissue found at surgery and the role of antecedent tendinopathy.
- Rupture most commonly occurs at the proximal insertion (inferior pole avulsion), less commonly mid-substance, and least commonly at the tibial tubercle.
- Systemic factors (corticosteroids, chronic renal failure with secondary hyperparathyroidism, diabetes, inflammatory arthropathy, fluoroquinolones) impair collagen quality and underlie spontaneous and bilateral ruptures.
Clinical Presentation
- Sudden pop or giving way during eccentric loading (jump landing, missed step, fall), with immediate inability to weight-bear or extend the knee.
- Always ask about systemic risk factors and prior anterior knee symptoms (tendinopathy) or steroid/fluoroquinolone use.
- Palpable infrapatellar gap below the inferior pole of the patella.
- Loss of active knee extension — the patient cannot perform a straight-leg raise or hold the extended knee against gravity (extensor lag). Note: an intact medial/lateral retinaculum may permit weak, painful extension in a partial tear, which is a diagnostic trap.
- High-riding patella (patella alta) palpable proximally, often with a haemarthrosis.
Always examine the contralateral knee and screen other tendons (quadriceps, Achilles) if a systemic cause is suspected.
Investigations
- Lateral view is key — shows patella alta and quantifies it with the Insall-Salvati ratio (tendon length / patella diagonal length; normal 0.8-1.2; over 1.2 = alta).
- The Caton-Deschamps index (articular surface to anterosuperior tibia / patellar articular length; normal about 1.0) is an alternative that is independent of patellar tendon length and is preferred when the tendon itself is abnormal or after TKA.
- Look for an avulsed bony fragment from the inferior pole or tubercle, and exclude patella fracture.
rapid, dynamic, bedside confirmation of tendon discontinuity, gap size and partial-versus-complete tears; highly operator dependent.
the most sensitive modality. Defines the level of rupture (proximal/mid/distal), completeness, gap size, tissue quality and concomitant injury — most useful in equivocal, partial, chronic or atypical cases. Not required when the clinical triad is unequivocal.
Differential Diagnosis of Acute Loss of Knee Extension
GAPDiagnostic Triad of Patellar Tendon Rupture
Hook:A ruptured tendon leaves a GAP
Management
Complete patellar tendon rupture is a surgical injury — conservative treatment leaves a permanent extensor lag and severe functional disability. Only genuinely partial tears with intact active extension (preserved retinaculum, able to straight-leg raise) may be treated non-operatively in a brace with the knee in extension. The principles are: repair early, restore patellar height, augment friable tissue, and mobilise early to avoid stiffness.
complete rupture, presenting within 2-3 weeks before retraction and scarring set in.
- Midline anterior incision; identify and debride the ragged tendon ends and the inferior pole.
- Place locking Krackow sutures (heavy non-absorbable, e.g. #5 / #2 high-strength) in each limb of the tendon, taking a 4-5 cm purchase.
- Reattach using 3 vertical transosseous patellar tunnels OR 2-3 suture anchors in the inferior pole. Biomechanically these are equivalent for ultimate load to failure; suture anchors show lower cyclic gap formation.
- Repair medial and lateral retinacular extensions of the tear.
- Confirm patellar height intra-operatively with a lateral image and compare with the contralateral side — avoid over-tightening.
Management Algorithm
Complications
Complications of Patellar Tendon Repair
Extensor Lag versus Fixed Flexion, and the Partial-Tear Trap
The diagnostic triad turns on "loss of active knee extension / extensor lag", and the topic explicitly warns that "an intact medial/lateral retinaculum may permit weak, painful extension in a partial tear" - the very trap that produces the missed diagnosis in the chronic-neglected scenario. But it never defines an extensor lag or explains how to avoid being fooled.
- Extensor lag (an ACTIVE deficit). An extensor lag is the difference between the passive extension a joint can be pushed to and the active extension the patient can achieve and hold against gravity. A lag means the extensor mechanism cannot deliver the force even though the joint reaches full extension passively - the signature of an extensor-mechanism disruption. It is distinct from a fixed flexion deformity, in which the joint cannot be passively extended (a mechanical/capsular block, not an extensor-mechanism problem).
- How a partial tear or intact retinaculum masks the injury. The medial and lateral retinacula run alongside the patellar tendon and can transmit some extensor force even when the central tendon is torn. A patient with a partial central tear, or a complete tear with intact retinacula, may therefore weakly extend the last few degrees or briefly hold a straight-leg raise, so a cursory examination looks reassuring - exactly how a complete rupture gets mislabelled a "sprain" (the chronic-neglected viva).
- How to avoid the trap. Test extension against gravity AND resistance through the full arc from a flexed start, not just the terminal few degrees; a patient who cannot initiate extension from a flexed knee, or who has any extensor lag, has a functionally significant disruption. Palpate for the infrapatellar gap, compare patellar height with the other side, and if any doubt remains obtain ultrasound or MRI - never accept partial active extension as excluding a rupture.
Q: A patient with knee trauma can weakly extend the last few degrees - can you exclude a patellar tendon rupture? A: No. An extensor lag (full passive extension but deficient active extension held against gravity) signals an extensor-mechanism disruption, and the retinacula can transmit enough force for weak terminal extension or a brief straight-leg raise even with a complete central tear - the classic reason a rupture is missed as a "sprain". Test extension against gravity and resistance through the whole arc from flexion, feel for the infrapatellar gap, compare patellar height, and image (US/MRI) if in doubt. (Contrast with a fixed flexion deformity, where the joint cannot be extended even passively.)
The Krackow Locking Suture
The repair technique specifies "locking Krackow sutures... taking a 4-5 cm purchase", but never explains what the Krackow stitch is or why it, rather than a simple suture, is the workhorse for reattaching tendon to bone.
- What it is. The Krackow stitch runs a series of interlocking loops down each side of the tendon, each loop grasping the collagen bundles so the suture locks onto the tissue rather than sliding through it. The two limbs (one down each side) exit distally as strong strands to pass through bone tunnels or tie to anchors.
- Why it is used. A ruptured patellar tendon usually fails through degenerate, friable "mop-end" tissue that a simple or mattress suture cheese-wires straight out of under load. The Krackow's locking loops distribute the load along a long 4-5 cm purchase and give a high load-to-failure, which is what permits a secure reattachment and early protected motion.
- How it fits the construct. Each Krackow limb is delivered through the three vertical transosseous patellar tunnels or tied to inferior-pole suture anchors (biomechanically equivalent for ultimate load, with anchors resisting cyclic gapping better). It is the same locking principle used to repair the quadriceps and Achilles tendons and in tendon transfers.
- The caveat. Even a perfect Krackow relies on the tendon substance, so when the tissue is very poor, chronic or deficient the repair must still be offloaded with augmentation (suture tape/cerclage) as covered above.
Q: Why is a Krackow locking suture used to reattach the patellar tendon rather than a simple stitch? A: The tendon fails through degenerate "mop-end" tissue that a simple/mattress suture pulls straight out of. The Krackow runs interlocking loops that grasp the collagen down a 4-5 cm purchase, giving high load-to-failure so the repair holds for early protected motion; the two limbs pass through transosseous tunnels or anchors (equivalent ultimate load, anchors gap less). It is the same stitch used for the quadriceps and Achilles - and it still needs augmentation when the tissue is poor.
Guidelines, Registries & Global Practice
Global epidemiology
- Patellar tendon rupture is the rarest of the three extensor-mechanism injuries but predominates in active males under 40 worldwide; quadriceps tendon rupture predominates over 40.
- A substantial minority (up to roughly one-third) carry local or systemic risk factors; systemic disease underlies most bilateral ruptures across all populations.
Society guidance & consensus (side by side)
- Position on patellar tendon rupture
- No disease-specific guideline; consensus and review literature support early surgical repair of complete ruptures with anatomic restoration of patellar height.
- Position on patellar tendon rupture
- No dedicated BOAST; managed under knee/soft-tissue trauma principles — urgent diagnosis of extensor-mechanism disruption and timely surgical repair.
- Position on patellar tendon rupture
- Describes operative repair (transosseous suture / anchors) and augmentation techniques as standard for extensor-mechanism reattachment.
- Position on patellar tendon rupture
- EFORT Open Reviews synthesis (Tandogan 2022) endorses early surgical repair, augmentation for poor/chronic tissue, and allograft for complex/post-TKA reconstruction.
There is broad international consensus rather than divergent national guidelines: complete rupture is a surgical injury, repaired early, with augmentation and reconstruction reserved for poor tissue, chronic and post-arthroplasty cases.
Registry note
- No dedicated patellar tendon rupture registry exists. Relevant signal comes from arthroplasty registries (NJR, AJRR, AOANJRR), where extensor-mechanism disruption is a recognised, function-limiting complication after TKA that drives revision and poorer patient-reported outcomes — the population in which reconstruction (and allograft) is most often required.
High- vs limited-resource practice variation
- Well-resourced settings: ready access to MRI, suture anchors, suture-tape internal brace and tissue-bank allograft (Achilles / full extensor-mechanism) for complex reconstruction.
- Limited-resource settings: reliance on clinical diagnosis and plain radiographs, transosseous tunnel repair with heavy suture and stainless-steel cerclage wire, and autograft (hamstring, contralateral BTB) where allograft is unavailable or unaffordable — all of which deliver good outcomes when applied to the correct indication.
Controversies & Areas of Uncertainty
- Suture anchors vs transosseous tunnels. Biomechanically equivalent for ultimate load, with anchors showing less cyclic gap formation; clinical outcomes are similar. Choice remains surgeon preference and tissue quality, not high-level clinical evidence.
- Routine augmentation in acute repair. Augmentation (suture tape / cerclage) is intuitively protective and allows accelerated rehabilitation, but a systematic review found primary repair with and without augmentation gave comparable Lysholm scores in good-quality acute tears — so universal augmentation in healthy young tendon is debated, while it is clearly favoured for poor tissue, chronic and post-TKA cases.
- Timing thresholds. "Acute" (under 2-3 weeks) versus "chronic" is a useful heuristic, but the true point at which retraction mandates reconstruction varies with tissue quality rather than a fixed day count.
- Allograft vs autograft for chronic/post-TKA reconstruction. Achilles allograft gives the best comparative scores in post-TKA series, but allograft cost, availability and incorporation risk keep hamstring and quadriceps autograft techniques relevant, especially in younger patients and resource-limited settings.
- Accelerated vs protected rehabilitation. Early motion reduces the dominant complication (stiffness) but must be balanced against re-rupture risk; the optimal protocol, and how much augmentation should change it, is not standardised.
- Bilateral simultaneous rupture: staged vs single-stage repair. No comparative trials exist; the decision is individualised around rehabilitation feasibility, comorbidity and social support.
Exam Viva Scenarios
Practise clinical reasoning and management decisions out loud
“A 30-year-old basketball player lands from a jump and feels a pop in his knee. He has immediate swelling and cannot stand up. On examination, there is a palpable defect below the patella and he cannot perform a straight leg raise. X-ray shows an Insall-Salvati ratio of 1.5. How do you assess and manage this injury?”
“You are called to the Emergency Department to see a 42-year-old man who has presented unable to walk after a fall down stairs. He reports that both knees 'gave way' simultaneously as he tried to catch himself. On examination, he has bilateral palpable infrapatellar gaps and cannot perform a straight leg raise on either side. He is obese (BMI 36) and on further history reveals he has had a renal transplant 3 years ago and is on long-term immunosuppression (prednisone 10mg daily, tacrolimus). He also has chronic kidney disease stage 3 and secondary hyperparathyroidism. X-rays of both knees show bilateral patella alta with Insall-Salvati ratios of 1.4 on the right and 1.6 on the left. How do you approach this patient's management and what are the specific considerations?”
“You are seeing a 38-year-old manual laborer in your clinic who was referred from a rural area. He sustained a patellar tendon rupture 9 months ago when he fell from a ladder at work. He was initially seen at a local hospital where he was told he had a 'knee sprain' and was given a knee brace and told to rest. He never had surgery. Over the past 9 months, he has been unable to work and has severe difficulty with stairs and standing from a seated position. He walks with a significant limp and cannot perform a straight leg raise. On examination, you note significant quadriceps atrophy, patella alta (the patella is palpable very high in the suprapatellar region), a large gap below the patella, and he has developed a significant extensor lag - he can achieve about 20° of active extension but cannot fully extend or hold the leg straight. His passive range of motion is 20° to 110° (he has developed a fixed flexion contracture). X-rays show an Insall-Salvati ratio of 2.1 indicating severe patella alta. The patellar tendon stump is retracted and scarred to the fat pad on MRI, and there is significant quadriceps muscle contracture and fatty infiltration. He is desperate to return to work and wants surgical reconstruction. How do you counsel him and what are the surgical options?”
Diagnostic Triad
- 1. Palpable Gap (Infra-patellar)
- 2. Loss of Active Extension
- 3. Patella Alta (X-ray)
Radiology
- Insall-Salvati: T/P
- Normal: 1.0 (0.8-1.2)
- Greater than 1.2 = Alta (Tendon Rupture)
- Less than 0.8 = Baja (Quad Rupture)
Key Evidence
Extensor mechanism ruptures — contemporary review
- Native patellar tendon ruptures occur in younger patients than quadriceps tendon ruptures
- Up to one-third of patients have local or systemic risk factors; these are more frequent in bilateral disruptions
- Complete disruptions should be repaired surgically; augmentation is advised for poor tissue quality, chronic tears or defects
- High rates of return to work/sport are reported with re-rupture rates under 5%
- Extensor mechanism injuries after TKA have inferior outcomes and warrant augmentation in all cases
Patellar tendon restoration — systematic review of repair vs reconstruction
- 23 studies, 738 patients across acute, chronic, post-TKA and post-ACLR settings
- Acute repairs achieved mean post-operative Lysholm scores of 84 to 99.5
- In the primary setting, primary repair with and without augmentation gave comparable Lysholm scores
- Cerclage augmentation alone yielded the lowest Lysholm scores and should not be relied upon as the sole construct
- In post-TKA patients, allograft reconstruction (KSS 79-88) outperformed autograft (KSS 70)
Suture anchor vs transosseous tunnel — biomechanical meta-analysis
- Systematic review and meta-analysis of 7 biomechanical studies, 128 cadaveric specimens (66 suture anchor, 62 transosseous)
- Suture anchor fixation produced significantly lower cyclic gap displacement than transosseous tunnels
- No significant difference in ultimate load to failure between the two techniques
- Both remain widely used and acceptable constructs in clinical practice
Chronic patellar tendon reconstruction after TKA — graft comparison
- 21 chronic post-TKA reconstructions in three groups of 7 (Achilles allograft, quadriceps+semitendinosus autograft, full extensor mechanism allograft)
- Mean extensor lag improved from about 50 degrees to about 3 degrees; mean KSS rose from 44.7 to 78.9
- Achilles allograft (KSS 87.7) significantly outperformed autograft (KSS 70.0)
- Authors conclude Achilles tendon allograft should be considered the gold-standard repair for chronic post-TKA lesions
Return to play after patellar tendon repair in professional athletes
- 103 professional baseball, basketball, American football and soccer athletes after patellar tendon repair
- 79 of 103 (76.7%) successfully returned to play
- American football athletes had the lowest return-to-play rate and the shortest adjusted careers
- Soccer athletes showed short-term performance decline that recovered to baseline by seasons 2-3
Hamstring double-row reconstruction for post-TKA patellar tendon rupture
- 14 patients (mean age 68.1) with post-TKA patellar tendon rupture reconstructed using ipsilateral semitendinosus and gracilis double-row technique
- Median Caton-Deschamps index improved from 1.8 to 1.25 (p = 0.014)
- Median active knee-extension loss fell from 25 degrees to 5 degrees
- Tegner-Lysholm and Kujala scores significantly improved at mean 3.8-year follow-up
References
According to PubMed, the evidence cards above are drawn from the following verified sources:
- Tandogan RN, et al. Extensor mechanism ruptures. EFORT Open Rev. 2022. PMID 35638613. DOI
- Fortier LM, et al. Patellar tendon restoration techniques: a systematic review. Eur J Orthop Surg Traumatol. 2024. PMID 39212690. DOI
- Imbergamo C, et al. Failure rates of suture anchor vs transosseous tunnel for patellar tendon repair: meta-analysis. Orthop J Sports Med. 2022. PMID 36035892. DOI
- Lamberti A, et al. Surgical options for chronic patellar tendon rupture in TKA. Knee Surg Sports Traumatol Arthrosc. 2016. PMID 27815584. DOI
- Nguyen MT, Hsu WK. Performance-based outcomes following patellar tendon repair in professional athletes. Phys Sportsmed. 2019. PMID 31291548. DOI
- Akgun E, et al. Double-row hamstring reconstruction for patellar tendon rupture after TKA. Medicine (Baltimore). 2024. PMID 38669383. DOI