Popliteus Anatomy

Origin. The tendon arises inside the joint, from the anterior fifth of the popliteal sulcus (groove) on the lateral aspect of the lateral femoral condyle, anterior and distal to the fibular collateral ligament (FCL). It is intracapsular but extrasynovial: a synovial reflection invests it and keeps it outside the joint fluid space while it lies inside the capsule.

The two numbers. In quantitative cadaveric mapping the popliteus femoral attachment averages 18.5 mm from the FCL femoral attachment, and the FCL itself attaches approximately 1.4 mm proximal and 3.1 mm posterior to the lateral epicondyle. These two numbers are the anatomical foundation of every anatomical posterolateral corner reconstruction. Reproducing the 18.5 mm separation is what distinguishes an anatomical reconstruction from a non-anatomical sling.

Through the hiatus. From its femoral attachment the tendon runs posteriorly, medially and distally, passing deep to the FCL. It then traverses the popliteal hiatus, the gap in the coronary (meniscotibial) attachment of the lateral meniscus through which it reaches the posterior compartment. The popliteomeniscal fascicles, usually described as anteroinferior, posterosuperior and posteroinferior, bridge the hiatus, tether the tendon to the lateral meniscus and form the borders of the hiatus.

What a lesion at the hiatus is worth. Stäubli's arthroscopic series found popliteus-system lesions in 95 per cent of acute and 85.7 per cent of chronic ACL-deficient knees, but also in 18.7 per cent of 107 knees with intact cruciates: nearly one in five. A frayed or detached fascicle seen at the hiatus is therefore a common finding and not automatically the patient's problem. It earns treatment only when the history, the dial test and the lateral meniscal behaviour agree with it.

The popliteofibular ligament. The PFL arises from the musculotendinous junction of popliteus and runs inferolaterally to the posteromedial downslope of the fibular styloid. It usually has anterior and posterior divisions, the posterior being the larger and stronger. It converts popliteus from a purely femorotibial structure into a femoro-fibular one, it is a primary restraint to tibial external rotation, and it is the structure most consistently disrupted in a PLC injury.

Belly and insertion. The triangular belly arises from the tendon and expands medially and distally to insert on the posterior surface of the tibia above the soleal line, over a triangular area on the posteromedial proximal tibia. Cadaveric work also describes slips to the posterior horn of the lateral meniscus and the posterior capsule, and in a substantial proportion of knees to the fibula and the posterior cruciate ligament region.

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The corner around it. Seebacher described the posterolateral corner in three layers, and the popliteus complex sits in the deepest:

• Superficial - the iliotibial band anteriorly and biceps femoris posteriorly
• Middle - the patellar retinaculum and the patellofemoral and patellomeniscal ligaments
• Deep - the lateral capsule, FCL, popliteus tendon, PFL, and the arcuate and fabellofibular ligaments

LaPrade defined the three primary structures within it: the FCL, the popliteus tendon and the PFL.

Sesamoid and capsular thickenings. A small sesamoid, the cyamella, lies in the popliteus tendon in a minority of knees and is occasionally mistaken for a loose body on radiographs. The arcuate and fabellofibular ligaments are variable posterolateral capsular thickenings. The fabellofibular ligament runs from the fabella to the fibular styloid and is present when a fabella is present.

Nerve supply. Popliteus is supplied by a branch of the tibial nerve, root values L4, L5 and S1.

The recurrent course. This is the classic exam point. The branch descends over the posterior (superficial) surface of the muscle, winds around its lower border, and enters the muscle on its deep (anterior) surface. The route reflects the embryological rotation of the limb bud: popliteus develops on the anterior (flexor) aspect and rotates posteriorly, carrying its nerve around the muscle's edge. It is the standard developmental explanation and worth stating.

Why the route matters. Because the nerve enters the deep surface, it is at risk in a posterior approach to the tibia or a deep posterolateral dissection that strips the muscle off the tibia from its anterior aspect. The injury is functionally minor but a source of posterolateral pain. The branch also sends articular twigs to the knee joint and the superior tibiofibular joint, by Hilton's law, and this is one route by which popliteus pathology refers pain diffusely to the posterolateral knee.

Denervation is near-silent. The medial hamstrings and gracilis also internally rotate the tibia, so isolated popliteus denervation matters for pain and for the loss of a dynamic PLC contributor rather than for measurable weakness.

Testing. There is no reliable isolated clinical test of popliteus strength. Resisted tibial internal rotation with the knee flexed loads it, but also loads semitendinosus, semimembranosus, gracilis and sartorius. What is tested clinically is the ligamentous function of the popliteus complex: the dial test, the external rotation recurvatum test, the posterolateral drawer and the reverse pivot shift.

Arterial supply. The tendon and the belly draw on several vessels:

• Middle genicular artery - a direct posterior branch of the popliteal artery that pierces the oblique popliteal ligament to supply the intra-articular structures, including the popliteus tendon and the cruciates
• Inferior lateral genicular artery - runs deep to the FCL and along the lateral meniscus, supplying the tendon in the hiatus region
• Inferior medial genicular artery and direct muscular branches of the popliteal artery - the muscle belly
• Posterior tibial recurrent artery - the distal muscle near the tibial insertion

The genicular vessels in surgery. The inferior lateral genicular artery, running deep to the FCL and superficial to the lateral meniscus, is at risk in lateral meniscal surgery, posterolateral corner reconstruction and lateral meniscal root repair. The middle genicular artery is divided routinely in a posterior approach or a PCL inlay procedure and is a source of persistent bleeding if not controlled.

The popliteal artery. It lies immediately posterior to the popliteus belly, separated by only a thin fascial layer. That is why the posterior approach to the proximal tibia is a high-consequence exposure, and why an arthroscopic instrument driven through the posterior capsule at the popliteal hiatus can reach the vessel.

The vascular danger line. At the posterior joint line the popliteal artery lies only a few millimetres to about a centimetre behind the posterior capsule, closest at the posterior horn of the lateral meniscus and the intercondylar region. Knee flexion increases this distance, which is why all posterior arthroscopic work and all PCL tunnel drilling is performed with the knee flexed to at least 90 degrees.

Behind the muscle. Popliteus forms the floor of the lower popliteal fossa, and the popliteal artery, vein and tibial nerve lie directly on it. Plantaris and the two heads of gastrocnemius cross superficially. The oblique popliteal ligament lies on the popliteus fascia proximally, and that fascia is a strong layer continuous with the tendinous expansions of semimembranosus.

Deep to it. The muscle lies on the posterior surface of the proximal tibia and the posterior capsule of the knee, with the superior tibiofibular joint laterally.

Around the tendon. The FCL lies superficial to the tendon, which passes beneath it. The lateral femoral condyle and lateral meniscus lie deep, and the popliteal hiatus and lateral compartment lie anterior.

The common peroneal nerve. It lies posterolateral to the tendon, running along the posteromedial border of the biceps femoris tendon toward the fibular neck, and it is the structure most at risk in any posterolateral corner surgery. It crosses the fibular neck roughly 2-4 cm distal to the tip of the fibular head, then divides into superficial and deep branches as it passes through peroneus longus.

Mechanism. The injury follows a varus force to a hyperextended knee, a posterolaterally directed blow to the anteromedial tibia, a twisting injury, or a knee dislocation. PLC injuries are rarely isolated: the great majority occur with a cruciate injury, most commonly the PCL, and frequently both.

The nerve at injury. A common peroneal nerve injury is present at the time of injury in a substantial minority. Its presence should raise suspicion of a severe corner injury or a spontaneously reduced knee dislocation.

Classification. Hughston grades the injury by lateral gapping; Fanelli grades it by the pattern of laxity and the structures torn.

• Hughston - grade I, 0-5 mm; grade II, 6-10 mm; grade III, more than 10 mm
• Fanelli type A - increased external rotation only: injury to the popliteofibular ligament and popliteus tendon
• Fanelli type B - increased external rotation plus mild varus opening: the above plus attenuation of the FCL
• Fanelli type C - increased external rotation plus marked varus opening: disruption of the popliteus tendon, popliteofibular ligament, FCL and lateral capsule

The clinically useful version. An isolated FCL injury behaves differently from a complete corner injury, and only the latter reliably causes rotatory instability.

Grades I and II. Isolated grade I and II injuries are treated non-operatively, in extension for 3-4 weeks with protected weight-bearing and progressive rehabilitation.

Grade III. These are operative, and this is the point candidates get wrong: a complete PLC injury treated non-operatively is a recognised cause of chronic instability and cruciate graft failure. Acute injuries (within 2-3 weeks) are treated by repair of avulsions with augmentation, or by primary reconstruction. The published outcomes consistently favour reconstruction over isolated repair, because repaired attenuated tissue stretches out.

Combined injuries. Address the corner at the same sitting as the cruciate. Reconstructing the cruciate alone in a PLC-deficient knee predictably fails.

Chronic injuries and the osteotomy. A chronic injury needs anatomical reconstruction. A varus mechanical axis or a varus thrust mandates consideration of a proximal tibial opening-wedge osteotomy, either as a first stage or concurrently. A proportion of patients become sufficiently asymptomatic after the osteotomy alone that the ligament reconstruction becomes unnecessary, a point worth making because it changes the counselling.

Slope. An opening-wedge osteotomy done anteromedially increases posterior tibial slope, which is favourable in a PCL-deficient knee and unfavourable in an ACL-deficient one. Plan the wedge geometry deliberately.

Reconstruction Techniques

Rehabilitation. After reconstruction:

• Non-weight-bearing or protected weight-bearing in extension for 6 weeks
• No active hamstring contraction early in a combined PCL and PLC reconstruction, because the hamstrings pull the tibia posteriorly and load the graft
• Avoid tibial external rotation loading for at least 3-4 months
• Return to pivoting sport at 9-12 months, and only with restored strength and no varus thrust

Popliteus tendon dysfunction after total knee arthroplasty. The patient has persistent posterolateral knee pain after an otherwise satisfactory total knee arthroplasty, often with a painful snap or catch felt laterally at around 30-45 degrees of flexion, and pain on stairs or on rising from a chair. It is an under-recognised cause of the unhappy but well-aligned, well-fixed, uninfected total knee.

Mechanisms. The tendon can be caught by any of the following:

• An oversized or overhanging femoral component, particularly one too large in the anteroposterior or mediolateral dimension, so the posterolateral flange impinges on the tendon
• A retained posterolateral osteophyte rubbing the tendon
• Prominent cement in the posterolateral corner
• Overstuffing of the lateral compartment or excessive component lateralisation
• Tibial component overhang posterolaterally
• Iatrogenic partial or complete popliteus tendon division during the lateral release or during posterior osteophyte removal

Exclusion first. Rule out infection (inflammatory markers and aspiration), loosening, instability, malrotation, and referred pain from the hip or spine. Do not diagnose popliteus impingement in a knee you have not properly worked up.

Confirming it. On examination there is focal posterolateral tenderness at the joint line, with pain reproduced by resisted internal rotation of the tibia and by the arc of flexion in which the snap occurs. Radiographs show component size, overhang and retained osteophytes; CT assesses component rotation; ultrasound can sometimes demonstrate the dynamic snap. A local anaesthetic injection into the popliteus tendon sheath that gives temporary relief supports the diagnosis and predicts benefit from release.

Treatment. Non-operative treatment is activity modification, physiotherapy and an image-guided injection. The operation is arthroscopic popliteus tenolysis or tenotomy, with removal of any retained osteophyte or prominent cement. If the cause is component oversizing or malrotation, revision may be required: a tenotomy will not fix a mechanically wrong component.

How thin the evidence is. The two papers this rests on (Allardyce 1997, Barnes 1995) are both indexed as case reports, and between them they report no denominator at all: no cohort, no failure rate, no follow-up interval. They establish that the mechanism exists and that release has relieved it in the patients described. They do not establish how often it works.

The sacrifice. It is usually tolerated, but that is a mechanical argument rather than a measured outcome. The cruciates are already sacrificed or substituted and the prosthesis provides the constraint, so the tendon's rotational function is largely redundant. No published series has followed these knees for rotational instability, so present it to the patient as reasoning rather than data.

Prevention. Size the femoral component accurately, remove posterolateral osteophytes carefully under vision with the knee flexed, avoid cement extrusion posterolaterally, and protect the popliteus tendon during a lateral release.

Popliteus tendinopathy and tenosynovitis. This is a condition of runners, especially downhill runners, and cyclists. It is an eccentric overload: popliteus decelerates internal femoral rotation and resists anterior femoral translation on the tibia during downhill running. The pain is posterolateral, worse downhill and on descending stairs, with tenderness at the popliteus tendon just posterior to the FCL in the figure-of-four position.

Telling it apart. Unlike iliotibial band syndrome, the tenderness is posterior to the lateral epicondyle rather than over it. Unlike a lateral meniscal tear, there is no true joint-line tenderness or mechanical locking. Management is load modification, avoiding downhill running, eccentric loading and correction of running mechanics, with ultrasound-guided injection for refractory cases.

Rupture and avulsion. These are rare in isolation and usually part of a posterolateral corner injury or a knee dislocation. Isolated avulsion from the lateral femoral condyle is described, typically in contact sport, and presents with posterolateral pain and a subtle increase in external rotation at 30 degrees. Treatment is anatomical reattachment acutely, and reconstruction if chronic and symptomatic.

The hypermobile lateral meniscus. Disruption of the popliteomeniscal fascicles leaves a lateral meniscus that can be displaced anteriorly into the joint by an arthroscopic probe. The patient has mechanical lateral symptoms (catching, giving way, a sense of something moving), often with a normal MRI, because the meniscus itself is not torn.

A genuinely missed diagnosis. Suspect it when convincing lateral mechanical symptoms meet a normal scan, and probe the lateral meniscus deliberately at arthroscopy in the figure-of-four position. Treatment is arthroscopic repair of the popliteomeniscal fascicles or capsular plication to stabilise the meniscus.

Snapping popliteus. A cause of painful lateral snapping, from the tendon subluxating over the lateral femoral condyle or the fibular head, or from a cyamella. Distinguish it from a snapping biceps femoris tendon over the fibular head, a snapping iliotibial band over the lateral epicondyle, a discoid lateral meniscus and popliteus impingement after arthroplasty.

Popliteus in arthroplasty balancing. The popliteus tendon is a flexion-space lateral structure. In a fixed valgus knee the sequential lateral release may involve the iliotibial band (an extension-space structure), the posterolateral capsule (extension), the popliteus tendon (flexion) and the lateral collateral ligament (both).

Release what matches the tight space. Releasing the popliteus tendon in a knee that is tight only in extension creates flexion instability, which is far harder to manage than the deformity being corrected; iatrogenic division during a lateral release or posterior osteophyte removal does the same if the tendon was contributing to the lateral flexion space. Reassess the gaps after every step. Many surgeons deliberately preserve the popliteus tendon during valgus correction for exactly this reason, preferring to pie-crust the iliotibial band and the posterolateral capsule.