GC230 Knee Sport Injuries: Part 5
Knee sport injuries Part 5 covers posterior cruciate ligament (PCL) injuries, characterized by disruption of the PCL typically due to a posterior-directed force on the proximal tibia, resulting in posterior knee instability.
This lecture (Part 5 of the Knee Sport Injuries series) covers extensor mechanism injuries of the knee — specifically quadriceps tendon rupture and patellar tendon rupture. These are relatively rare but clinically devastating injuries because the extensor mechanism is the sole system that allows active knee extension. Missing the diagnosis means a patient who cannot straighten their knee, cannot walk properly, and if the diagnosis is delayed, surgical outcomes worsen dramatically.
Learning Objectives (derived from slide content) [1]:
- Understand the anatomy and function of the extensor mechanism
- Recognise, investigate, and manage quadriceps tendon rupture
- Recognise, investigate, and manage patellar tendon rupture
Core Anatomy and Function of the Extensor Mechanism
The quadriceps is one of the three anti-gravity muscles of the lower limb and functions as the primary extensor of the knee joint [1]
The three anti-gravity muscles of the lower limb are gluteus maximus (hip), quadriceps (knee), and triceps surae/soleus (ankle). They resist gravity during standing and walking.
The extensor mechanism of the knee is composed of the quadriceps tendon, patella, and patellar tendon [1]
Think of it as a chain: quadriceps muscle → quadriceps tendon → patella (sesamoid bone) → patellar tendon → tibial tuberosity. A break anywhere in this chain = loss of active knee extension.
The patella is a sesamoid bone located within the extensor mechanism. It increases the biomechanical advantage of the quadriceps by increasing the distance between the quadriceps contraction and the rotational axis of the knee joint. [1]
Why does distance matter? Torque = Force × Moment Arm. The patella pushes the quadriceps tendon anteriorly (away from the centre of knee rotation), increasing the moment arm and therefore the torque produced for any given force of quadriceps contraction. Without the patella, you'd need significantly more quadriceps force to extend the knee.
Rupture of the extensor mechanism (either a quadriceps tendon rupture, a patella fracture, or a patellar tendon rupture) results in an inability to actively extend the involved knee [1]
Key Clinical Principle
Any disruption of the extensor mechanism chain — whether quadriceps tendon rupture, patella fracture, or patellar tendon rupture — presents with the SAME cardinal sign: inability to perform active straight leg raise / active knee extension. The location of tenderness and X-ray findings help you distinguish which component is disrupted.
The quadriceps is composed of four muscles — vastus lateralis, vastus intermedialis, vastus medialis, and rectus femoris, each has its own inserting tendon. [1]
The tendons of these four muscular elements fuse to form a common quadriceps tendon 2 cm proximal to the upper pole of the patella. [1]
The quadriceps tendon has a three-layered arrangement:
- Superficial: rectus femoris
- Intermediate: vastus medialis and vastus lateralis
- Deep layer: vastus intermedialis [1]
Memory aid: "RML-VL/VM-VI" — Rectus femoris Most superficiaL, then the two Vasti (Medialis/Lateralis), then Vastus Intermedialis deepest.
There is anatomical variation — the tendon can be mono-, bi-, tri-, or quadrilaminar [1].
The average thickness of the quadriceps tendon is 10 mm. [1]
The blood supply to the quadriceps tendon is through three arcades: the medial, the lateral, and the peripatellar. [1]
There is a zone of relative hypovascularity in the quadriceps tendon between 1 cm to 2 cm from the superior pole of the patella. [1]
This hypovascular zone is a common site of quadriceps tendon rupture. [1]
Why does hypovascularity matter? Areas with poor blood supply undergo degenerative changes more readily because they cannot mount adequate healing/repair responses. This is the same principle behind why Achilles tendon ruptures occur in a "watershed zone" 2-6 cm from the calcaneal insertion.
Degenerative tendinopathy is common in patients suffering from quadriceps tendon rupture (64%) [1]
The incidence of degenerative tendinopathy increases with increasing age of the patient [1]
The patellar tendon is a mono-layer structure located between the lower pole of the patella and the tibial tuberosity. [1]
It is covered by a layer of paratenon, similar to the Achilles tendon. [1]
It blends on either side with the retinacula of the knee joint. [1]
This is clinically important — even with a complete patellar tendon rupture, if the retinacula are intact, the patient may retain some (weak) ability to extend the knee, potentially masking the diagnosis.
The average thickness of the patellar tendon is 5 to 6 mm. [1]
Compare: quadriceps tendon is ~10 mm thick. The patellar tendon is about half the thickness.
Blood supply: anterior aspect from inferior-medial and inferior-lateral genicular arteries; posterior aspect from the anastomotic arch in Hoffa's fat pad. [1]
There is a hypovascular zone in the proximal portion of the patellar tendon. [1]
Degenerative tendinopathy is common in older individuals and in patients who engage in activities involving repetitive jumping and running. [1]
This is "jumper's knee" (patellar tendinopathy) — chronic overload at the proximal patellar tendon predisposes to eventual rupture.
Quadriceps Tendon Rupture
Rare injury: 1.37 / 100,000 per year [1]
More common in > 40 years old; mean age: 57 years [1]
Male : Female = 4.2 : 1 [1]
Key contrast with patellar tendon rupture: Quadriceps tendon rupture occurs in OLDER patients (> 40, mean 57), while patellar tendon rupture occurs in YOUNGER patients (3rd–4th decade). This is a classic exam discriminator.
"Spontaneous rupture" in at-risk subjects: chronic renal failure, rheumatoid arthritis, diabetes, gout, steroids [1]
Traumatic: indirect at the time of eccentric contraction of the quadriceps against resistance [1]
What is eccentric contraction? The muscle is lengthening while trying to contract — e.g., landing from a jump, going down stairs, stumbling. This puts maximal stress on the tendon.
Simple fall (61.5%), fall from stairs (23.4%), sporting activities (6%) [1]
Severity: Spontaneous (32.1%), minimal trauma (39.2%), high energy trauma (28.6%) [1]
This means most quadriceps tendon ruptures happen with minimal or no significant trauma — the underlying tendon is already degenerative.
Prior steroid injection, systemic use of corticosteroid, fluoroquinolone antibiotics [1]
High Yield – Fluoroquinolones and Tendon Rupture
Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin) are a well-established risk factor for tendon rupture, particularly in the elderly, those on steroids, and those with renal impairment. The mechanism involves inhibition of tenocyte proliferation and collagen synthesis, plus direct toxic effects on tendon matrix. This is a favourite pharmacology crossover exam question [1].
Acute pain, localized tenderness in the rupture area, a palpable gap may be found in the supra-patellar region, loss of active extension and a significant difference between active and passive extension (i.e., a big extensor lag) [1]
What is an extensor lag? The difference between the range of passive extension (examiner extends the knee) and active extension (patient tries to extend). A large extensor lag = something in the extensor mechanism is disrupted.
Clinical signs may be masked by haemarthrosis and intact retinaculum, leading to a delayed diagnosis in 10–50% of cases [1]
Exam Trap: Delayed Diagnosis
Haemarthrosis and intact retinaculum can mask quadriceps tendon rupture, causing delayed diagnosis in 10–50% of cases. If a patient has knee swelling after a fall and seems unable to do straight leg raise but you can't feel a gap — don't assume the extensor mechanism is intact. Get imaging [1].
X-ray findings:
Patella baja, obliteration of the normal quadriceps tendon shadow, supra-patellar calcific densities, supra-patellar mass [1]
Why patella baja? The quadriceps tendon is ruptured above the patella. The intact patellar tendon pulls the patella distally (inferiorly), resulting in a low-riding patella = patella baja (or patella infera).
Contrast with patellar tendon rupture where you see patella alta (high-riding patella) because the quadriceps pulls the patella superiorly when the patellar tendon is disrupted below.
| Feature | Quadriceps Tendon Rupture | Patellar Tendon Rupture |
|---|---|---|
| Age | > 40 (mean 57) | 3rd–4th decade |
| Site of tenderness | Supra-patellar | Infra-patellar |
| Palpable gap | Above patella | Below patella |
| X-ray patella position | Patella baja | Patella alta |
| Most common rupture site | Hypovascular zone 1–2 cm above patella | Inferior pole of patella (avulsion) |
USG and MRI can confirm the diagnosis, differentiate partial from complete tear, and locate the exact tear site [1].
Incomplete rupture: can be treated non-operatively, depending on extent of tear and patient's occupation and sports activity. Immobilization of knee in full extension for six weeks [1]
Why full extension? This position takes tension off the extensor mechanism, allowing the torn fibres to heal without being pulled apart.
Acute complete rupture: early surgical repair should be advised because:
- The quadriceps apparatus begins to retract in the first few days
- After days or weeks, retraction makes apposition of the torn tendon ends difficult and increases tension on the suture line
- Can be done by open surgery or endoscopic assisted operation [1]
High Yield – Why Early Repair Matters
The quadriceps retracts rapidly after rupture. Within days, end-to-end repair becomes difficult. Within weeks, direct repair may be impossible and tendon lengthening or reconstruction is needed. This is why early diagnosis is critical [1].
Chronic complete tear:
- Proximal stump retracts after a few days
- Apposition of the torn tendon ends becomes difficult
- There will be increased tension in the repair site
- Lengthening of tendon is required in cases of chronic rupture [1]
Loss of knee flexion range, quadriceps muscle atrophy, and extensor mechanism weakness are common after quadriceps tendon rupture [1]
Patellar Tendon Rupture
Avulsion with or without bone from the inferior pole of the patella (most common), mid-substance tear, avulsion from the tibial tuberosity [1]
The inferior pole avulsion is most common because this is where the hypovascular zone is, and where maximal stress concentrates.
Tensile overload of the extensor mechanism with sudden quadriceps contraction with knee in a flexed position (e.g., jumping sports, missing step on stairs) [1]
This is the classic "jumper's knee rupture" — a basketball player going up for a rebound, or someone stumbling on stairs.
Patients suffering from systemic disease, such as DM, SLE, RA, chronic renal disease; patients suffering from patellar tendinopathy; patients who receive corticosteroid injection [1]
Note the overlap with quadriceps tendon rupture risk factors: systemic diseases affecting connective tissue, steroids, and pre-existing tendinopathy.
Patients report a popping sensation at the time of rupture, infrapatellar pain, immediate knee swelling, difficulty in weight bearing [1]
Signs: gross haemarthrosis, obvious bruise in the infrapatellar region, localized tenderness at the infrapatellar region, palpable gap below the inferior pole of the patella, reduced range of motion of knee, failure to perform active straight leg raise, unable to maintain a passively extended knee [1]
Clinical Pearl
"Failure to perform active straight leg raise" and "unable to maintain a passively extended knee" are the two key functional tests. If you passively extend the patient's knee and then let go, the knee will drop into flexion — this is pathognomonic of complete extensor mechanism disruption [1].
X-ray findings: patella alta, avulsion fracture from the lower pole of patella [1]
Patella alta because the quadriceps muscle (still attached to the intact quadriceps tendon) retracts the patella superiorly when the patellar tendon below is torn.
USG and MRI: help to locate the site of the tear; help to differentiate partial tendon rupture from complete tendon rupture [1]
Incomplete rupture with intact extensor mechanism: can be treated non-operatively, depending on extent of tear and patient's occupation and sports activity. Immobilization of knee in full extension for six weeks [1]
Acute complete rupture: early surgical repair [1]
Chronic complete rupture: patellar tendon reconstruction with a tendon graft [1]
Unlike chronic quadriceps tendon rupture (where lengthening may suffice), chronic patellar tendon rupture typically requires a tendon graft because the thin mono-layer patellar tendon retracts and degenerates, making direct repair impractical.
The potential complications include knee stiffness, quadriceps atrophy, re-tear, and infection [1]
| Feature | Quadriceps Tendon Rupture | Patellar Tendon Rupture |
|---|---|---|
| Incidence | 1.37/100,000/yr | < 1/100,000/yr |
| Age | > 40 years (mean 57) | 3rd–4th decade |
| Sex | M:F = 4.2:1 | M > F |
| Tendon structure | Multi-layered (3 layers) | Mono-layer with paratenon |
| Tendon thickness | ~10 mm | ~5–6 mm |
| Hypovascular zone | 1–2 cm above superior pole of patella | Proximal portion |
| Most common mechanism | Simple fall (eccentric loading) | Jump landing / missing stairs |
| Risk factors | CRF, DM, RA, gout, steroids, fluoroquinolones | DM, SLE, RA, CRF, patellar tendinopathy, steroids |
| Tenderness location | Supra-patellar | Infra-patellar |
| Palpable gap | Above patella | Below patella |
| X-ray | Patella baja | Patella alta ± avulsion # inferior pole |
| Delayed diagnosis rate | 10–50% | Lower (more obvious signs) |
| Incomplete tear Mx | Extension immobilization × 6 weeks | Extension immobilization × 6 weeks |
| Complete acute tear Mx | Early surgical repair | Early surgical repair |
| Chronic tear Mx | Tendon lengthening | Tendon graft reconstruction |
| Complications | Stiffness, atrophy, weakness | Stiffness, atrophy, re-tear, infection |
The lecture series (Parts 1–6) covers [3]:
- Introduction to knee sports injuries
- Ligament injuries (ACL, PCL, MCL, LCL, knee dislocation)
- Meniscus injuries
- Cartilage injury and osteochondral fracture
- Extensor mechanism injury (this lecture)
- Patellofemoral joint instability
The management principles from Part 1 apply here [3]:
Definitive management depends on whether the injured tissue can heal and whether it can restore normal function if allowed to heal
For extensor mechanism injuries:
- Incomplete tears → tissue CAN heal → non-operative (immobilization)
- Complete tears → tissue CANNOT heal on its own (tendon retracts) → surgical repair
- Chronic tears → tissue has retracted and scarred → reconstruction/lengthening
The DDx for extensor mechanism injury in the context of an acute knee presentation [2]:
- ACL tear: audible pop, immediate haemarthrosis, instability/giving way, positive Lachman/pivot shift — but can still extend the knee
- Meniscal tear: locking, clicking, joint line tenderness — can still extend the knee
- Patella fracture: similar presentation (inability to extend), palpable defect over the patella itself, X-ray shows fracture line through patella
- Patella dislocation: visible lateral displacement, apprehension test positive
Exam Intelligence
- "A 60-year-old man with CKD stumbles on stairs, acute knee pain, cannot do SLR, palpable gap above patella" → Quadriceps tendon rupture
- "A 30-year-old basketball player lands from a jump, pop in knee, patella riding high on X-ray" → Patellar tendon rupture
- "What drug class is associated with tendon rupture?" → Fluoroquinolones
- "Why is delayed diagnosis common in quadriceps tendon rupture?" → Haemarthrosis + intact retinaculum mask the gap and extensor lag
- "Patella baja vs patella alta — which condition for each?" → Baja = quadriceps tendon rupture; Alta = patellar tendon rupture
- "Why should complete rupture be repaired early?" → Tendon retracts within days, making end-to-end repair progressively more difficult
| Trap | Why Students Fall for It | Correct Answer |
|---|---|---|
| Confusing patella alta and baja | They mix up which tendon is torn | Alta = patellar tendon rupture (patella pulled UP); Baja = quadriceps tendon rupture (patella pulled DOWN) |
| Thinking quadriceps tendon rupture is a young person's injury | "Sports injuries = young people" | Quadriceps tendon rupture peaks at ~57 years; patellar tendon rupture is in younger patients |
| Forgetting fluoroquinolones as a risk factor | Not connecting pharmacology to orthopaedics | Fluoroquinolones cause tendon degeneration → rupture risk |
| Assuming all ruptures need surgery | Partial tears can be managed conservatively | Incomplete ruptures with intact mechanism → immobilization in extension × 6 weeks |
| Missing the diagnosis because patient can still partially extend | Intact retinaculum allows some extension | Always test SLR and ability to maintain passive extension; get imaging |
High Yield Summary
-
Extensor Mechanism = Quadriceps tendon → Patella → Patellar tendon → Tibial tuberosity
-
Any break in this chain → inability to actively extend the knee (cannot do SLR)
-
Quadriceps tendon rupture
- Older patients (> 40, mean 57), hypovascular zone 1–2 cm above patella, risk factors include CRF/DM/RA/gout/steroids/fluoroquinolones, supra-patellar gap, patella BAJA on X-ray, delayed diagnosis in 10–50% due to haemarthrosis/intact retinaculum
-
Patellar tendon rupture
- Younger patients (3rd–4th decade), most commonly avulses from inferior pole of patella, jumper's mechanism, infra-patellar gap, patella ALTA on X-ray ± inferior pole avulsion fragment
-
Management of both
- Incomplete: extension immobilization × 6 weeks
- Complete acute: early surgical repair (tendon retracts rapidly)
- Chronic: quadriceps = tendon lengthening; patellar = tendon graft reconstruction
-
Drug risk factors
- Corticosteroids (local or systemic), fluoroquinolones
-
Patella function
- sesamoid bone that increases moment arm of quadriceps → increases torque for knee extension
Active Recall - Knee Sport Injuries Part 5: Extensor Mechanism Injury
GC230 Knee Sport Injuries: Part 4
Knee sport injuries Part 4 covers posterior cruciate ligament (PCL) injuries and posterolateral corner injuries, including their mechanisms, clinical assessment, and management principles.
GC230 Knee Sport Injuries: Part 6
Sport-related knee injuries encompassing complex or multi-ligament injuries, including knee dislocations and combined ligamentous damage, requiring comprehensive evaluation and often surgical reconstruction.