Popliteus Anatomy

Origin — Intra-Articular

• Site: the popliteal sulcus (groove) on the lateral aspect of the lateral femoral condyle, at the anterior fifth of the sulcus.
• Relationship to the fibular collateral ligament (FCL): the popliteus tendon attaches anterior and distal to the FCL. 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.
• Character: the tendon is intracapsular but extrasynovial, invested by a synovial reflection that keeps it outside the joint fluid space while it lies inside the capsule.

Course — the Popliteal Hiatus

• From its femoral attachment the tendon runs posteriorly, medially and distally, passing deep to the fibular collateral ligament.
• It traverses the popliteal hiatus, the gap in the coronary (meniscotibial) attachment of the lateral meniscus through which the tendon reaches the posterior compartment.
• The popliteomeniscal fascicles — usually described as anteroinferior, posterosuperior and posteroinferior — bridge the hiatus, tethering the tendon to the lateral meniscus and forming the borders of the hiatus.
• What a lesion at the hiatus is worth as a sign. 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. So a frayed or detached fascicle seen at the hiatus is a common finding, not automatically the patient's problem, and it earns treatment only when the history, the dial test and the lateral meniscal behaviour agree with it.

The Popliteofibular Ligament

• Arises from the musculotendinous junction of popliteus and runs inferolaterally to attach to the posteromedial downslope of the fibular styloid process.
• Usually has anterior and posterior divisions, the posterior division being the larger and stronger.
• Functionally: it converts popliteus from a purely femorotibial structure into a femoro-fibular one, and it is a primary restraint to tibial external rotation. It is the structure most consistently disrupted in a PLC injury.

Muscle Belly and Insertion

• The muscle belly is triangular, arising from the tendon and expanding medially and distally.
• Insertion: the posterior surface of the tibia, above the soleal line, over a triangular area on the posteromedial proximal tibia.
• Additional attachments described in cadaveric work include slips to the posterior horn of the lateral meniscus, the posterior capsule, and in a substantial proportion of knees to the fibula and the posterior cruciate ligament region.

Related Structures

• Popliteal sesamoid (cyamella): a small sesamoid bone in the popliteus tendon, present in a minority of knees, occasionally mistaken for a loose body on radiographs.
• Arcuate ligament and fabellofibular ligament: 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

• Nerve: a branch of the tibial nerve.
• Root values: L4, L5, S1.
• The recurrent course — the classic exam point: the branch to popliteus descends over the posterior (superficial) surface of the muscle, then winds around its lower (inferior) border and enters the muscle on its deep (anterior) surface.
• Why this odd route: it 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 with it. It is the standard developmental explanation and worth stating.

Practical Consequences

• Because the nerve enters the deep surface, it is at risk during a posterior approach to the tibia or a deep posterolateral dissection that strips the muscle off the tibia from its anterior aspect.
• The branch also supplies articular twigs to the knee joint and to the superior tibiofibular joint — Hilton's law again, and one route by which popliteus pathology refers pain diffusely to the posterolateral knee.
• Isolated popliteus denervation is functionally near-silent because the medial hamstrings and gracilis also internally rotate the tibia. It 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 muscle 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

• 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 fibular collateral ligament and along the lateral meniscus, supplying the tendon in the hiatus region.
• Inferior medial genicular artery and direct muscular branches of the popliteal artery supply the muscle belly.
• The posterior tibial recurrent artery supplies the distal muscle near the tibial insertion.

Surgical Consequences

• The inferior lateral genicular artery is at risk during lateral meniscal surgery, posterolateral corner reconstruction and lateral meniscal root repair, running deep to the FCL and superficial to the lateral meniscus.
• The popliteal artery lies immediately posterior to the popliteus muscle belly, separated by only a thin fascial layer. This is the reason 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 middle genicular artery is divided routinely during a posterior approach or a PCL inlay procedure and is a source of persistent bleeding if not controlled.

The Vascular Danger Line

• At the level of the posterior joint line, the popliteal artery lies only a few millimetres to about a centimetre posterior to the posterior capsule, closest at the level of 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.

Superficial (Posterior) to the Muscle

• The popliteal artery, vein and tibial nerve — the muscle forms the floor of the lower popliteal fossa and the vessels lie directly on it.
• The plantaris muscle and the two heads of gastrocnemius, crossing superficially.
• The oblique popliteal ligament, which lies on the popliteus fascia proximally.
• The popliteus fascia, a strong layer continuous with the tendinous expansions of semimembranosus.

Deep (Anterior) to the Muscle

• The posterior surface of the proximal tibia and the posterior capsule of the knee.
• The superior tibiofibular joint laterally.

At the Tendon

• Superficial: the fibular collateral ligament, under which the tendon passes.
• Deep: the lateral femoral condyle and the lateral meniscus.
• Anterior: the popliteal hiatus and the lateral compartment.
• The common peroneal nerve lies posterolateral, running along the posterior border of the biceps femoris tendon toward the fibular neck — the structure most at risk in any posterolateral corner surgery.

Structures at Risk in Posterolateral Surgery

• Common peroneal nerve — posterior to the biceps tendon, crossing the fibular neck approximately 2-4 cm distal to the fibular head tip. It must be identified and protected in every PLC reconstruction and in every fibular head osteotomy or drilling.
• Popliteal artery and vein — immediately posterior to the popliteus belly.
• Inferior lateral genicular artery — deep to the FCL along the lateral meniscus.
• Lateral meniscus posterior horn and its root — immediately adjacent to the popliteal hiatus.
• Anterior tibial artery — passes through the interosseous membrane just distal to the tibiofibular joint; at risk during transfibular tunnel drilling.

Mechanism and Epidemiology

• Mechanism: a varus force to a hyperextended knee, a posterolaterally directed blow to the anteromedial tibia, a twisting injury, or a knee dislocation.
• Rarely isolated: the great majority of PLC injuries occur with a cruciate injury — most commonly the PCL, and frequently both.
• Common peroneal nerve injury is present at the time of injury in a substantial minority, and its presence should raise suspicion of a severe corner injury or a spontaneously reduced knee dislocation.

Classification

• Hughston grading (by lateral gapping): grade I, 0-5 mm; grade II, 6-10 mm; grade III, more than 10 mm.
• Fanelli classification:
  • Type A: increased external rotation only — injury to the popliteofibular ligament and popliteus tendon.
  • Type B: increased external rotation plus mild varus opening — the above plus attenuation of the FCL.
  • 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.

Management Principles

• Grade I and II isolated injuries: non-operative, in extension for 3-4 weeks with protected weight-bearing and progressive rehabilitation.
• Grade III injuries: operative. 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 (within 2-3 weeks): repair of avulsions with augmentation, or primary reconstruction. The published outcomes consistently favour reconstruction over isolated repair, because repaired attenuated tissue stretches out.
• Chronic: anatomical reconstruction, preceded or accompanied by a corrective osteotomy if there is a varus mechanical axis or a varus thrust.
• Combined injuries: address the corner at the same sitting as the cruciate. Reconstructing the cruciate alone in a PLC-deficient knee predictably fails.

Reconstruction Techniques

Rehabilitation

• Non-weight-bearing or protected weight-bearing in extension for 6 weeks.
• No active hamstring contraction early in 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

• Presentation: 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

• An oversized or overhanging femoral component, particularly a component that is too large in the anteroposterior or mediolateral dimension, so the posterolateral flange impinges on the popliteus 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.

Diagnosis

• Exclusion first: 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.
• Examination: focal posterolateral tenderness at the joint line, pain reproduced by resisted internal rotation of the tibia and by the arc of flexion in which the snap occurs.
• Imaging: radiographs for component size, overhang and retained osteophytes; CT for component rotation; ultrasound can sometimes demonstrate the dynamic snap.
• Diagnostic injection: a local anaesthetic injection into the popliteus tendon sheath giving temporary relief supports the diagnosis and predicts benefit from release.

Management

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

Isolated Popliteus Tendinopathy and Tenosynovitis

• Population: runners, especially downhill runners, and cyclists.
• Mechanism: eccentric overload of the popliteus, which decelerates internal femoral rotation and resists anterior femoral translation on the tibia during downhill running.
• Presentation: posterolateral knee pain, worse downhill and on descending stairs, tender at the popliteus tendon just posterior to the FCL in the figure-of-four position.
• Discriminators: 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: load modification, downhill running avoidance, eccentric loading and correction of running mechanics; ultrasound-guided injection for refractory cases.

Popliteus Tendon Rupture and Avulsion

• Rare in isolation; 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: anatomical reattachment acutely; reconstruction if chronic and symptomatic.

Popliteal Hiatus and the Hypermobile Lateral Meniscus

• The popliteomeniscal fascicles tether the lateral meniscus at the hiatus. Their disruption produces a hypermobile lateral meniscus — a lateral meniscus that can be displaced anteriorly into the joint by an arthroscopic probe.
• Presentation: mechanical lateral symptoms — catching, giving way, a sense of something moving — often with a normal MRI, because the meniscus itself is not torn.
• This is a genuinely missed diagnosis. Suspect it in a patient with convincing lateral mechanical symptoms and a normal scan, and probe the lateral meniscus deliberately at arthroscopy in the figure-of-four position.
• Treatment: 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 (popliteal sesamoid).
• Distinguish 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 Total Knee 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).
• The critical rule: release the structure that 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 you were correcting.
• Many surgeons deliberately preserve the popliteus tendon during valgus correction for exactly this reason, preferring a pie-crusting technique of the iliotibial band and the posterolateral capsule.