Anatomy in practice: the Popliteus muscle.

ABSTRACT

Examination for trigger points in the popliteus muscle involves palpation of its muscle belly and proximal tendon of attachment. Review of the popliteus muscle in situ revealed its location on the floor of the popliteal fossa and the association of overlying soft tissues and neurovascular bundles. While the premise in clinical texts is that this muscle is easily accessible, the clinical anatomy of popliteus highlights that palpation is not as straightforward as often depicted. Woodley S, Mercer S (2006): Anatomy in practice: the Popliteus muscle. New Zealand Journal of Physiotherapy 34(1): 25-29.

INTRODUCTION

The differential diagnosis for posterior knee pain can be complex, and vascular and neurologic pathology needs to be considered alongside musculoskeletal disorders (Muche and Lento 2004). From a musculoskeletal perspective, popliteus has been implicated in complaints of posterior knee pain that is exacerbated with activities such as crouching and either walking or running, downhill or downstairs. One suggestion regarding the pattern of pain referral from popliteus is that it arises from an active trigger point located within the middle of its muscle belly. In addition, if symptomatic, the popliteal tendon and the region of its femoral attachment may be tender when palpated. When describing techniques of palpation it has been proposed that the tibial attachment, the upper lateral end of popliteus and the femoral attachment are palpable (Chaitow and Walker DeLany 2002, Travell and Simons 1999). Interestingly, the topography of the muscle is not included in these clinical descriptions. As the morphology of the popliteus muscle in situ is rarely discussed in relationship to musculoskeletal assessment of the knee region, the purpose of this paper was to present the clinical anatomy of this muscle.

Morphology

Popliteus is described as a thin or flattened triangular shaped muscle. Its broad muscle belly attaches medially to the posterior surface of the tibia above the soleal line, tapering to an apex as it approaches the knee joint (Figure 1) (Grant and Basmajian 1965, Hollinshead 1969, Gardner et al 1975). From their distal attachment, the fascicles of popliteus pass superiorly and laterally, running beneath the arcuate ligament (Last 1948, Watanabe et al 1993). Becoming tendinous, it then passes between the fibrous and synovial layers of the knee joint capsule, and continues upwards towards its proximal insertion. Attachment proximally is into the lateral surface of the lateral condyle of the femur, below the attachment of the lateral collateral ligament (Figure 2) (Frazer 1940, Hollinshead 1969, Watanabe et al 1993). A bursa is found deep to the tendon where it passes between the lateral collateral ligament and the lateral meniscus.

[FIGURES 1-2 OMITTED]

Clear associations between popliteus and surrounding structures have been identified. A feature of its fleshy attachment to the tibia is that it is covered by dense fascia, which is particularly thick medially, thereby acting as an aponeurosis for the semimembranosus muscle (Grant and Basmajian 1965, Moore and Dalley 2006). Associations have also been observed with the joint capsule, lateral meniscus, posterior cruciate ligament, ligaments of Wrisberg and Humphrey, oblique popliteal ligament, the arcuate ligament complex, and to the head of the fibula (Figure 1) (Jones et al 1995, Kimura et al 1992, Last 1948, Last 1950, Terry and LaPrade 1996, Tria et al 1989, Ullrich et al 2002, Wadia et al 2003, Watanabe et al 2003).

Function

Popliteus provides posterolateral stability to the knee joint and aids in stabilising the lateral meniscus and controlling tibial rotation (Jones et al 1995, Muche and Lento 2004, Nyland et al 2005, Ullrich et al 2002). This muscle is not thought to contribute significantly to flexion of the knee joint (Fuss 1989, Kaplan 1962, Moore and Dalley 2006). However, it has been suggested that popliteus aids in unlocking and internally rotating the knee joint when initiating flexion, and that it may control antero-posterior motion of the lateral meniscus throughout the motion of flexion (Fuss 1992, Last 1948, Moore and Dalley 2006). In instances when the knee adopts a static flexed position, popliteus is also thought to assist the posterior cruciate ligament in preventing anterior displacement of the femur on the tibia (Moore and Dalley 2006). As this paper is concerned with the morphology of the popliteus muscle, readers interested in the function of this muscle are referred to a recent review (Nyland et al 2005).

Popliteal Fossa

When contemplating palpation of popliteus, the muscle must be considered in situ situ. From a physiotherapy perspective, familiarity with the anatomy of the popliteal fossa is therefore necessary.

Located at the back of the knee, the popliteal fossa is a diamond shaped area which may be divided into an upper and a lower triangle. The upper triangle is bounded medially by the semimembranosus muscle and overlying semitendinosus tendon. The short head of biceps femoris, overlaid and fused with the long head of biceps femoris, forms the lateral border (Figure 3). These muscles and tendons embrace the proximal sides of the lower triangle which are comprised of the two heads of gastrocnemius and the very small plantaris muscle, which lies beneath the lateral head. The floor of the fossa is largely formed by the posterior aspect of the distal femur which is covered by fat, and the posterior capsule of the knee joint. The thick fascia covering the popliteus muscle completes the floor distally (Figure 4) (Grant and Basmajian 1965, Hollinshead 1969, Woodburne and Burkel 1988). Covering the fossa to form a roof are the dense, circularly arranged fibres of the fascia lata, which pass distally to become continuous with the deep fascia of the leg. It has been suggested that this overlying popliteal fascia is tensioned when the knee joint is extended (Gardner et al 1975).

[FIGURES 3-4 OMITTED]

The popliteal fossa contains numerous structures including the common peroneal and tibial nerves, popliteal artery and vein, posterior femoral cutaneous nerve, the genicular branch of the obturator nerve, the small saphenous vein, lymph nodes, bursae and fat (Figure 4). All of these various structures must be considered when attempting to palpate the popliteus muscle as they lie between the skin and the popliteus muscle (Hollinshead 1969, Gardner et al 1975, Woodburne and Burkel 1988)

In the midline, the floor is crossed vertically by the tibial nerve and popliteal vessels (Figure 4). The popliteal artery lies on the fascia covering the popliteus. In the upper part of the fossa, the lateral and medial superior genicular arteries arise from the popliteal artery, while the middle genicular artery arises behind the knee joint. Important to the therapist considering the popliteus muscle, the medial and lateral genicular branches pass medially and laterally over popliteus to run deep to their corresponding collateral ligaments before joining the arterial anastomosis around the knee joint. The terminal branches of the popliteal artery, the anterior and posterior tibial arteries, arise at the lower border of popliteus. Typically the lesser saphenous vein pierces the popliteal fascia, passing between the two heads of gastrocnemius to drain into the popliteal vein (Grant and Basmajian 1965, Hollinshead 1969, Gardner et al 1975, Woodburne and Burkel 1988, Moore and Dalley 2006).

In the upper lateral corner the common peroneal nerve passes close to the medial border of biceps femoris. This nerve follows the biceps tendon as it passes out of the fossa, over the lateral head of gastrocnemius, to the back of the head of the fibula (Figure 3). Located within the fossa, the tibial nerve lies in the midline on the popliteus muscle before passing distally, deep to the fibrous arch of the soleus muscle (Figure 4) (Hollinshead 1969).

Implications for Palpation

Palpation of the popliteus muscle must occur through the overlying structures of the popliteal fossa. Consequently the site of the midbelly trigger point (Chaitow and Walker DeLany 2002, Travell and Simons 1999) is buried deep beneath skin, subcutaneous tissue, deep fascia, the gastrocnemius muscle and the overlying dense fascia of the popliteus muscle (Figure 5). In addition the tibial nerve and popliteal vessels pass over the muscle (Figures 3 and 4).

[FIGURE 5 OMITTED]

Popliteus is considered to be most accessible at two locations--close to the lower medial end, and to the upper lateral end of the muscle belly (Figure 1). It has been proposed that the lower medial end of the muscle can be palpated directly between the semitendinosus tendon and the medial head of the gastrocnemius muscle (Figures 3 and 4) (Travell and Simons 1999). To access this area, once, and if it is possible that the medial head of gastrocnemius can be pushed laterally, contact with popliteus would be restricted by overlying skin, subcutaneous tissue, the crural fascia, and the overlying dense aponeurosis of the semimembranosus muscle (Figures 4 and 5). The upper, lateral end of popliteus is said to be best palpated as it crosses the knee joint just above the head of the fibula, between the tendon of biceps femoris and the lateral head of gastrocnemius (Travell and Simons 1999) (Figures 3 and 4). Laterally, the overlying skin, subcutaneous tissue, crural fascia, tendon of biceps femoris, common peroneal nerve and arcuate ligament complex would obstruct direct access to the muscle (Figures 1, 3-5).

Travell and Simons (1999) have also stated that when popliteus is involved in the complaint of posterior knee pain, patient examination will reveal tenderness of its tendon as well as the region over its tendinous attachment to the femur. When palpating in the area of its proximal attachment the presence of other local structures also require consideration. These include the lateral collateral ligament, lateral meniscus, the bursa deep to popliteus, a tendinous expansion from vastus lateralis, the tendon of biceps femoris, fascia lata, and the joint capsule (Figures 2 and 3).

CONCLUSIONS

Examination of the popliteus muscle in situ reveals those soft tissues that would hamper specific palpation of the popliteus muscle. In addition to the skin and subcutaneous tissue superficially, a medial approach encounters the substantial crural fascia, medial head of gastrocnemius and the dense aponeurosis of semimembranosus. Laterally, tissues such as the crural fascia, tendon of biceps femoris, common peroneal nerve and arcuate ligament impede direct access to the muscle. Potential pain generating structures such as the lateral collateral ligament, lateral meniscus, bursae and the joint capsule should also be considered when attempting to palpate the popliteal tendon near its femoral insertion. Physiotherapists assessing the posterior aspect of the knee joint should be aware of the morphology and relations of the popliteal muscle and its tendon.

ACKNOWLEDGEMENTS

The authors which to thank Mrs Shannon O'Neill, Mr Brynley Crosado and Mr Russell Barnett for the preparation of the material used to illustrate this paper. This material forms part of the teaching collection of the Department of Anatomy and Structural Biology at the University of Otago.

REFERENCES

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Hollinshead WH (1969): Anatomy for Surgeons: Volume 3. The Back and Limbs. London: Harper and Row.

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Key Points

* The in situ morphology of popliteus is complex as this musculotendinous unit is associated with, and attached to, numerous soft tissues and neurovascular bundles

* Popliteus is located deep, close to the floor of the popliteal fossa

* Physiotherapists considering palpation of popliteus need to have an awareness of the location of this muscle in relation to tissues which overlie and surround it.

Stephanie Woodley BPhty MSc

PhD student

Department of Anatomy & Structural Biology

University of Otago

Dunedin

New Zealand

Susan Mercer BPhty(Hons) MSc PhD

Department of Anatomy & Developmental Biology

School of Biomedical Sciences

The University of Queensland 

Australia

ADDRESS FOR CORRESPONDENCE

Stephanie Woodley, Department of Anatomy & Structural Biology, University of Otago, Dunedin, New Zealand. Dr Susan Mercer, Department of Anatomy & Developmental Biology, School of Biomedical Sciences, The University of Queensland, Australia Q 4072. Email: [e-mail]