|Year : 2021 | Volume
| Issue : 1 | Page : 35-40
Sheath of distal tendon of semimembranosus muscle and it's functional significance
Subhash D Joshi, Sharda S Joshi, Namrata Valimbe
Department of Anatomy, Sri Aurobindo Medical College and Post Graduate Institute, Indore, Madhya Pradesh, India
|Date of Submission||04-Oct-2020|
|Date of Acceptance||22-Feb-2021|
|Date of Web Publication||07-Apr-2021|
Dr. Subhash D Joshi
Department of Anatomy, Sri Aurobindo Medical College and Post Graduate Institute, Indore, Madhya Pradesh
Source of Support: None, Conflict of Interest: None
Introduction: Semimembranosus (SM) muscle is important in maintaining the stability of knee joint, and the distal end of it's tendon is not only complex but has been described differently in the available literature, which is quite perplexing. How a muscle with such divergent attachments can efficiently perform it's functions? Hence a detailed study of the distal semimembranosus tendon unit was undertaken. Material and Methods: One hundred lower limbs (Rt-50; lt.-50) were utilized for the present study. The posterior surface of SM was exposed and as the main tendon could not be traced to its insertion hence a vertical incision was made on the posterior surface of this tendon and this lead to the splitting of the tendon-sheath which exposed the thick shining distal tendon of SM. Results: Surprisingly, it was found that the main tendon of the SM (SMT) was surrounded by a fibrous sheath which was derived from the main muscle mass in the lower part of the thigh and in the majority of cases it was separated from it by a synovial space. Traced inferiorly it was observed that it is the sheath that spreads out to various divergent destinations described in the literature, whereas the main tendon gets attached to the groove on the posterior aspect of the medial condyle of tibia and a rough triangular area below the groove. Discussion and Conclusion: It is the nature's ingenuity that a single muscle with such divergent distal attachments can still efficiently perform its functions due to the formation of a sheath of distal tendon of SM.
Keywords: Distal tendon, expansions of SM, semimembranosus, sheath of SMT
|How to cite this article:|
Joshi SD, Joshi SS, Valimbe N. Sheath of distal tendon of semimembranosus muscle and it's functional significance. J Anat Soc India 2021;70:35-40
|How to cite this URL:|
Joshi SD, Joshi SS, Valimbe N. Sheath of distal tendon of semimembranosus muscle and it's functional significance. J Anat Soc India [serial online] 2021 [cited 2021 Oct 25];70:35-40. Available from: https://www.jasi.org.in/text.asp?2021/70/1/35/313158
| Introduction|| |
The posterior aspect of the knee, due to the clinical relevance of its musculoskeletal system is very important and the anatomists must have a definitive and precise understanding of the morphology of this region.
Semimembranosus muscle (SM) is a key component of the complex anatomy of the posteromedial aspect of knee joint. The distal tendon of semimembranosus has always attracted the curiosity and attention of orthopedic surgeons, radiologists, biomechanists, physical therapists and anatomists, but there is no clearly defined uniformity in the description of termination of the tendon of insertion of semimembranosus and it's nomenclature. A variable number of expansions of the distal tendon of SM have been described.
It had always been very intriguing to be convinced by the idea that in spite of so many divergent expansions and attachments of the distal tendon of SM muscle at its insertion, how can it so efficiently perform the movement of flexion at the knee joint? Why the pull of the contracting muscle should be lost or wasted by diverging to such attachments as oblique popliteal ligament (OPL), Fascia over the popliteus muscle, medial collateral ligament (MCL) of the knee joint, menisci, etc., and it's main tendon of insertion on the medial condyle of tibia?
The literature search revealed significantly inconsistent morphology of the distal semimembranosus tendon unit (SMTU). This prompted us to undertake a detailed study of SM muscle.
| Material and Methods|| |
One hundred lower limbs (Rt. 50, Lt. 50), which were used for the routine dissection by students over a period of few years were utilized for the present study. Hamstrings were traced towards the popliteal fossa. On the medial side, the tendon of semitendinosus was identified and reflected inferiorly exposing the posterior surface of the semimembranosus muscle. The neuro-vascular bundle present in the popliteal fossa was cleaned and removed to expose the posterior surface of the fibrous capsule of knee joint and the OPL, the fascia covering the popliteus muscle and in the upper medial part the Posterior Oblique Ligament (POL), MCL was also cleaned and exposed.
Having done this the thick tendon of SM was identified; but the main tendon that has to insert on tibia could not be seen. Hence, a vertical incision was made on the posterior surface of this tendon and this lead to the splitting of the tendon-sheath which exposed the thick shining distal tendon of SM [Figure 1]. After extending the incision inferiorly on the tendon sheath the main tendon could be traced inferiorly where it was found to continue: (a) as a thick tendon that bent at right angle to pass anteriorly deep to MCL in the groove on the posterior surface of medial condyle of tibia, and (b) the rest of it continued inferiorly and expanded to be attached to the rough triangular area below the Medial condyle of tibia [Figure 2].
|Figure 1: Dissection of the posterior aspect of right knee showing the Thick tendon of SM (red arrow) seen after splitting the sheath (cut edges shown with black arrows) which extends well above the joint line of the knee|
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|Figure 2: The main tendon of SM (red arrow) to bifurcate into two after crossing the knee joint. One part (black star) passes deep to the medial collateral ligament (cut edges of which are shown by yellow arrows) and gets attached to the groove on the posterior surface of medial condyle of tibia. The other part (yellow star) gets attached to the rough triangular area below the groove|
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Tendon sheath of SM was carefully traced distally where it was found to diverge to be continuous with: (a) OPL, (b) fascia covering the posterior surface of popliteus muscle [Figure 3] and [Figure 4], (c) merging with the posterior edge of the MCL and some fibres passing deep to it to be attached to the upper part of medial border of tibia (POL). Details were noted and Photographs were taken at various stages of dissection.
|Figure 3: Main tendon (red arrow); cut edge of the sheath indicated by black arrows. Oblique popliteal ligament shown by white interrupted arrow which is seen to be continuous with sheath of SM|
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|Figure 4: Posterior surface of left SM main tendon shown by red arrow. Edges of the split sheath shown by black arrows. Oblique popliteal ligament (yellow arrow) is seen to be continuous with the sheath. Green arrows indicate the fascia covering the popliteus mu|
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The posterior surface of the upper end of dry tibia were examined for the presence of groove on the posterior surface of the medial condyle and the rough triangular area below the groove.
| Results|| |
When the tendon of Semitendinosus is reflected downwards behind the knee then the grooved posterior surface of SM was clearly visualized. Slightly above the knee joint, the fleshy fibres are replaced by the thick strong tendon [Figure 1]. A vertical incision was made on the posterior surface of this tendon which opened up a sheath surrounding the main tendon which lies within it [Figure 5] and [Figure 6]. In the majority of cases, there was a shining synovial sheath surrounding the tendon [Figure 7]. In other cases, where the lining synovial membrane between the tendon and sheath was absent, the tendinous sheath was found to be adherent to the main tendon. One part of the main tendon passed almost at right angles deep to the MCL to be attached to the groove on the posterior surface of the medial condyle of tibia [Figure 8]. The other part of the tendon was found to be attached to the triangular area on the posterior surface of the medial condyle of tibia just below the groove [Figure 2]. Posterior surface of upper end of dry tibia were examined and it was found that in the majority there was prominent rough triangular area below the groove on the posterior surface of medial condyle. It showed prominent vertical ridges and also a large number of vascular foramina [Figure 9]a, [Figure 9]b, [Figure 9]c. The tendon sheath when dissected and traced distally was found to spread out to various destinations to form: (a) POL, (b) fascia over the popliteus muscle, (c) OPL and to merge with MCL [Figure 3] and [Figure 4]. At the level of the middle of thigh: on the anterior surface of the SM muscle fleshy fibers were observed, which when traced inferiorly were found to be flanking on either side of the centrally placed muscle mass and led to the formation of the sheath of SM, and the central part continued as the main tendon of the muscle [Figure 10] and [Figure 11].
|Figure 5: Dissection of the posterior aspect of right knee showing the Thick tendon of SM (red arrow) exposed after splitting the sheath (black arrows). Fascia over the popliteus muscle (yellow arrow) and the medial ligament of knee (white arrow) are also seen|
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|Figure 6: Dissection of the posterior aspect of left knee showing tendon of SM (red line) and cut edges of sheath (black arrows). Medial collateral ligament (yellow arrow)|
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|Figure 7: Dissection of the posterior aspect of left knee showing the SM tendon (red arrow) seen after splitting the sheath of the tendon (black arrows); delicate synovial membrane (yellow arrow) seen. This is also seen to wrap the tendon and its sheath (blue stars|
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|Figure 8: Dissection of the posterior aspect of left knee showing tendon of SM seen to divide into two: One part passes deep to medial collateral ligament (yellow arrows) of knee and the other proceeds in a straight line (red arrow) to posterior surface of tibia below the medial condyle. Cut edges of the sheath shown by black arrows. Black strip is placed deep to the tendon, between it and the sheath|
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|Figure 9: (a) Posterior surface of upper end of left tibia showing the insertion of SM by red arrow in the groove on the posterior surface of medial condyle of tibia and the rough triangular area below the groove. (b) Posterior surface of right tibia showing the insertion of SM by red arrows in the groove on the posterior aspect of medial condyle and the rough triangular area below it. (c) The groove on the posterior aspect of medial condyle of tibia marked by red and rough triangular area indicated by black triangle|
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|Figure 10: Central part of SM (red arrow) continues inferiorly as the main tendon, and the flanking part of the muscle on either side (black arrows) when traced inferiorly leads to the formation of the sheath of the SM tendon|
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|Figure 11: The central part of SM (red arrow) is seen to continue as a strong tendon and the muscle mass flanking on either side continues inferiorly as tendon sheath of SM|
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| Discussion|| |
In the present study, the main tendon of SM after crossing the knee joint was seen to divide into two parts: One part of the tendon passed almost at right angles deep to the MCL to be attached to the groove on the posterior surface of the medial condyle of tibia [Figure 2]. This is in agreement with the description by Gardner et al., Sinnatamby, Standring et al. Snell, and Benninger and Delamarter.
The other part of the main tendon was getting attached to the triangular area on the posterior surface of the medial condyle of tibia just below the groove [Figure 2] and [Figure 12]. It has been mentioned that the main tendon is attached to a tubercle on the posterior aspect of medial tibial condyle; and others have also mentioned that from the deep part of the tendon a short slip is attached to a tubercle below the groove, which in the present study was found to be in the form of a rough triangular area [Figure 9]a, [Figure 9]b, [Figure 9]c below the medial condyle of tibia. This is quite a prominent area of insertion on the posterior surface of tibia below the groove on the medial condyle of tibia, giving attachment to SM tendon, which has been shown in illustrations in books.,,[Figure 12]
|Figure 12: Posterior surface of left SM main tendon (red star) diverting into two limbs (red arrows)- the right curved one is lodged in the groove on the posterior surface of medial condyle and the vertical component continues inferiorly and expands to be attached to the rough area below the groove|
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Expansions of distal tendon of SM
While reviewing the literature we find that various workers have described the number of expansions of distal attachment of SM varying from three to eight, and the nomenclature of these expansions is also variable and confusing.
Moore stated that SMT divides distally into three parts.
Sinnatamby stated that the insertion of SMT diverges into three expansions: (1) passes forwards along the medial surface of the condyle deep to tibial collateral ligament, (2) expansion passes obliquely upwards to the lateral femoral condyle as OPL, and (3) forms a strong fascia over popliteus and reaches the soleal line of tibia.
In a comprehensive review article, Benninger and Delamarter have quoted the works of some authors,, who have described a total of five expansions of SM.
Moncayo et al. have also described that there are five tendinous expansions in the meniscocapsular complex of distal SM insertions.
Maeseneer et al. have stated that the anatomy of the distal SMT is complex and six different insertions can be identified on routine proton density and FE sequences at 3 T.
Kim et al. have described a tendinous branch of SM inserting into posterior horn of lateral meniscus in 43.2% of knees dissected, This is in addition to 5 already known insertional branches i.e., six attachments.
Beltran et al. described five tendinous insertions and additionally two more attachments to the posterior horns of medial and lateral meniscus, i.e., a total of seven attachments; and have further stated that the different tendinous arms of SM complex are intimately related to the POL and the arcuate ligament. The POL (Ligament of Winslow) originates from the adductor tubercle of the femur.
Turman et al. have mentioned eight consistent insertions of SM, distal to the main common tendon. They have stated that as the insertion of SM tendon is extra capsular, it is not typically visualized during arthroscopy.
LaPrade et al. described SMTU and have stated that there are eight consistent posterior attachments of the SM distal to the main common tendon at the knee.
In the present study, we found that the fleshy fibres commenced at the level of the middle of the thigh. This is in agreement with the observation wherein it has been stated that the muscle fibres of SM commence about the middle of the thigh. Whereas other authors have stated that the tendon of SM becomes muscular in the upper part of the thigh and the tendon of insertion begins about the middle of the thigh.,
In the present study, it was observed that the fleshy fibres, that were present on the anterior surface of the muscle, when traced inferiorly were seen to be flanking on either side of the centrally placed muscle mass [Figure 10] and led to the formation of the sheath of SM. The centrally placed muscle fibres continued as the main tendon of the muscle [Figure 11] in the lower part of the thigh.
Gardner, Gray and O'Rhahilly have not described the sheath of SM but mentioned that it consists of a Superficial part which turns upwards and laterally as the OPL of the knee joint; the remainder forms the fascia of popliteus and is attached to the medial border and soleal line of tibia.
In the literature reviewed, we found only Warren and Marshal who described the SMTU as having two main insertions and a tendon sheath. The division between the tendon fibres going directly to the bone and the fibres of the sheath splaying out into the posteromedial part of the capsule is usually clear.
Loredo et al. who have studied the postero-medial aspect of the knee with MR imaging and correlated it with gross anatomy and concluded that there are: five arms of insertions, the third arm is derived mainly from the tendon sheath and blends with the posteromedial capsule.
In the present study, we have observed that the main tendon gets attached to the groove on the posterior surface of medial condyle of tibia and the rough triangular area below it. All the expansions were seen to be derived from the tendon sheath of SM rather than the main tendon as is described by different workers. The sheath was seen to be spreading out in various directions to their destination viz. POL. MCL, popliteal fascia and OPL.
Kim et al. are of the opinion that the multiple insertions of SM at the knee contribute to the medial stability of the knee specially in flexion, and when combined with the action of popliteus muscle they contribute together to maintain the posterior stability.
Beltran et al. discussing the biomechanics have mentioned that the intricate distal insertion of SM in the posteromedial aspect of the knee and biceps in the posterolateral aspect of the knee allow these two muscles to become very important stabilizing structures during knee flexion. The dynamic synergy of the SM complex and the biceps femoris also provides controlled stability of the flexed knee during internal and external rotation. Conceptualization of SM and biceps femoris complexes as horse reins attached to the medial and lateral aspect of the capsule adds to the understanding of these functions.
SM stabilizes the posterior capsule through OPL and acts synergistically with the popliteus muscle through the fibrous extension. It contributes to the stability of posteromedial aspect of the joint through its intertwining with POL They further mention that the different structures of the SM complex act as a suction cup placed over the posteromedial aspect of the knee as seen from inside the joint with the knee in flexion.
Kim et al. state that SM actively pulls the posterior horns of the medial and lateral meniscus, protecting them from being crushed between femoral condyles and the medial and lateral tibial plateau during knee flexion.
Maeseneer et al. mention that in knee extension the SMT prevent valgus, whereas in knee flexion it prevents external rotation.
Schache et al. discussing the biomechanics of the Human Hamstring Muscles during Sprinting mention that as peak musculotendon force and strain for long head of Biceps femoris, Semitendinosus and SM occurred around the same time during terminal swing phase, the biarticular hamstrings are at greatest risk of injury. SM produced the highest peak force, absorbs and generates the most power and performs the largest amount of positive and negative work.
| Conclusion|| |
After a detailed study, it can be stated that nature has with its ingenuity provided an excellent solution to this complexity of distal attachment of SM muscle by allowing the main tendon of insertion to pass through a sheath, which is also derived from the same muscle (and is mostly separated by a synovial space), that the sheath spreads out to their various destinations. The main tendon passes (a) deep to the MCL to be attached to a groove on the posterior surface of the medial condyle of tibia and (b) to a rough triangular area at the upper end of the posterior surface of the shaft of tibia below this groove.
The sheath gets wide purchase over all these areas of attachment of the distal tendon of SM and probably has a stabilizing effect on the central tendon to exert its unhindered pull over its insertion to bring about flexion at the knee joint as well as provide stability to the joint.
Thus it makes it amply clear that the architecture of the muscle can be resolved into a sheath of SM that spreads out to various divergent attachments and surrounds the main tendon that can work independently on the tibia during the movements at the knee joint.
Thus SM besides providing strength to the posteromedial part of the knee joint can also function unhindered due to the presence of it's sheath to produce the flexion at the knee joint and medial rotation of leg.
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Conflicts of interest
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]