Popliteus Muscle . An Anatomical Study

Results: The popliteus muscle presented a scalene-like triangle shape and its medial base was covered by the superficial expansion of the semimembranosus tendon. The length of its base was 79,2±12,8 mm with a superior and inferior side that measured 58.0±7.4 mm and 101.7±11.7 mm respectively. The popliteus muscle innervation was supplied by the tibial nerve (TN) by the arising of two branches in 22 cases (95.7%). The first branch arose proximal to the inter articular line and presented a length of 91.1 mm. The second branch arose distally to the inter articular line with a length of 48.4±1.3mm. The third was observed in just one case (4.3%) Its length was 108±1.1mm and it arose distally to the inter articular line.


Introduction
The popliteus muscle (PM), located in the deep layer of the posterior compartment of the leg, is immediately proximal to the soleus line.Its function is still being studied.Some authors have pointed out that the PM has two proximal tendon-ligament origins, which were fused to form a muscular body in its distal insertion in the posterior surface of the tibia, the action of these fibers allow the knee to unlock while walking [1,5,15,18].However, recent biochemical studies [3,6,8] emphasize in the origin of PM is fundamentally tibial with its tendon insertion in the popliteal sulcus (PS) in the femur and its tendonligament complex in the structures that make up the posterolateral corner of the knee (PLC) fulfilling its function of a primary dynamic stabilizer of the external rotation between the tibia and the femur and of a secondary restrictor in posterior cruciate ligament (PCL) injuries [7].
Initial anatomic studies [1,5,15,18] made distinctly qualitative descriptions about PM morphology.Hwang [9], was the first one to describe quantitatively the dimensions of PM.Its scalenelike triangle shape configures a posterolateral acute angle over the junction with the PT, whereas its posteromedial angle becomes larger.Thereby the PM, short and thick, curses from the tibia to its insertion in the femur.
Furthermore, the PT and its conformation of fiber bundles that interlace to confer stability to PLC of the knee, has reported a variability in its length prowling a range between 36-54.5 mm [11,12].This characteristic is involved in reparative and reconstructive processes from the structures that make up the PLC of the knee.
PM innervation is given by the tibial nerve (TN), however a full and detailed information about the morphometric characteristics of its innervation is still very limited, which are of big importance during surgical, esthetical and reconstructive procedures and in the leg and foot flexor musculature pain relief [2].
The aim of this study was to characterize the finding of anatomy of PM and its innervation, from the evaluation of fresh knees of a mestizo-raced population, predominant in Latin America.

Materials and Methods
This cross-sectional study was made in 23 knee segments 14 right knees and 9 left knees coming from 14 men and 9 women aged 67±22 years.This knees coming from supracondylar amputations for ischemic pathologies as a part of the anatomopathological study developed in the Industrial University of Santander.For the sample selection, it was used as inclusion criteria: knees from adults that belonged to the mestizo race group (mixture between Europeans and Natives), without any anatomical alteration caused by the amputation, without surgery records, traumatisms or related pathologies.This investigation was approved by the Institutional Review Board (ethics committee).
For the PM exposure, a midline incision over the popliteal fossa that compromised skin, subcutaneous tissue and fascia was made.The plantaris muscle, the lateral gastrocnemius and the medial gastrocnemius were visualized releasing from their proximal insertions.The characteristics of the morphologic and morphometric expression of the PM and its insertion in the tibia and the femur, as well as the characteristics of the motor branches for the PM that arise from the TN having as reference the interarticular line of the knee (IAL) and its relation with the popliteal artery (PA) were determined.
The measurements were made with a digital gauge and the specimens were photographed with a digital camera.The findings were saved and underwent statistical analysis.The continuous variables were described with average and standard deviation, the nominal ones in percentages.The statistical variables were proved with a chi-squared test (χ 2 ), student's T-test and the test of acceptance of an alpha error until 5%.A result of p<0.05 was considered significant.

Results
Popliteus Muscle origin was located in the posterior surface of the tibia, immediately distal to the interarticular line (IAL).PM was presented elongated, thin and shaped like a scalene triangle; its medial base was covered by the superficial expansion of the semimembranosus tendon.The length of its base was 79.2±12.8mm with a superior and inferior side that measured 58.0±7.4 mm and 101.7±11.7 mm respectively, without presenting statistically significant differences related to its side of presentation or gender (p=0.18).From its muscular insertion in the tibia, the popliteus tendon (PT) went from medial to lateral following an ascendant trajectory to its femoral insertion.The PT presented a length of 38.3±3.9 mm with a thickness of 3.1±0.5 mm in the middle of its course, without presenting statistically significant differences related to its side of presentation (p=0.21)(Figure 1

. A).
At the level of the PM myotendinous junction, ligamentous bands oriented towards the head of the fibula (Popliteal-fibular ligament), the posterior horn of the lateral meniscus (Popliteal-meniscal ligament) and the joint capsule arose; those characteristics were observed in the sample.(Figure 1. B).
Popliteus muscle innervation was supplied by the tibial nerve (TN) by the arising of two branches in 22 cases (95.7%) and three branches in one case (4.3%).The first branch arose proximal (50.3±1.5mm) to the IAL and presented a length of 91.1 mm (Figure 2.A).This branch was divided in 4 segments according to its anatomic relation with the PA: The first segment (N 1 or pre-vascular) with a length of 49.9±5.8mm,was located lateral to PA.The second segment (N 2 or vascular) adopted a profound trajectory to PA and measured 17.8±3.7mm.The third segment (N 3 or post-vascular) of this branch, with a length of 33.4±30.8mmfollowed a descendant and medial trajectory in relation to the PA and was accompanied by the middle genicular vessels and they all reached the superior border of the PM.Finally, this nervous branch cursed sub facially over the posterior surface of the PM, presenting in this distal segment (N 4 or sub facial) a length of 19.5±4.4mm.The motor point of the branch form TN was located 35.2±0.5mmfrom the IAL.
The second branch for the PM arose from TN distally to the IAL (27.52±0.7mm)with a length of 48.4±1.3mm; it was presented as a common trunk with the branch for the posterior tibialis muscle in 20 cases (87%).From its arising, it cursed with a lateral trajectory to the TN and descended over the posterior surface of the PM until it reached the middle segment of its inferior border and thus giving this muscle its innervation along with the posterior tibialis muscle (Figure 2.B).In two specimens (8.7%) a common trunk with branches for the PM and for the soleus muscle was observed.Its motor point was located 47±1.3mm distal to the IAL.The third branch for the PM was observed in just one case (4.3%) arising from the TN in a common trunk with branches for the plantaris, soleus and posterior tibialis muscles.Its length was 108±1.1mmand its motor point was located 22.4±0.8mmdistal to the IAL.

Discussion
PM is the most proximal muscle located in the deep layer of the posterior surface of the leg.Its triangular shape allows it to adopt an oblique trajectory having contact with the tibia and the femur to contribute in the stabilization of the knee [6,7].The PM dimensions that were found in our study allowed its qualitative characterizing as a triangular, elongated and thin muscle, similar to what was reported in initial anatomical studies [1,8].Meanwhile, Hwang [9] in his study about the PM shape and innervation developed in Korean cadaveric specimens, describes the PM as a short and thick muscle.It is only in our series and Hwang's studies that a morphometric description of the PM dimensions is made.Previous reports [1,8,14,15] made only a qualitative description about its morphology finding elongated and thin shape for the PM as described in our study.This divergence in the PM morphological characterization could be explained due to the variation in height of the studied individuals, proper of each demographic group.
PT length, width and thickness found in our work are located in a medial range to what was reported in eastern countries studies [11,20].In the mentioned works, the PT presented a length between 36.0 mm-40.4mm, with a PT width of 6.3 mm-10.1 mm and a thickness in a range of 2.4 mm-3.6 mm.On the other hand, the PT length found in our series is smaller than the one reported in the American population.Fineberg et al. [4] and LaPrade et al. [12] reported the PT length of 42 mm-54.4mm, establishing length ranges for the PT way above our findings (Table 1).This morphological feature can be possibly explained by the fact that Americans have a larger size relative to the Latin American population and by the differences in measurement criteria of the PT length among different authors [4,12,13].PM biomechanical functions have been established according to the direction of the vector formed between its origin and insertion bone structure surfaces.The first studies that reported the PM anatomy, pointed out two tendon-ligament proximal origins, one in the head of the fibula and the other one in the lateral condyle of the femur; and its insertion in a muscular body over the posterior surface of the tibia.It is showed that the PM serves as an anchor to unlock the knee during walking having a function of inverse contraction [1,5,8,15].On the other hand, various recent studies have determined the PM implication as a primary dynamic stabilizer of the external rotation between the tibia and the femur taking into consideration its distal origin and proximal insertion [7,16,17].
About the PM innervation, Hwang [9] found a branch coming from the TN that was always proximal to the IAL and it was the only branch for the PM.In our study up to 3 branches were found innervating the PM.The first branch, always proximal as Hwang [9] reported, was divided into four segments with the PA.Besides the importance of the PM morphological characteristics mentioned, it is worth mentioning that the knowledge of the PM innervation must be considered in the infiltrative processes made for pain relief in certain patients with leg and feet flexor spasticity secondary to stroke [2].If the location of the PM branches and its relation with the branches for other muscles of the posterior layer are not considered, iatrogenic effects could be generated and compromise the knee's posterolateral stability in patients without any records of direct or rotation trauma over the knee.
The morphological characteristics of the popliteus muscle and its innervation patterns found in our study differ from what was reported in previous studies.Our study is innovative since it does a detailed description of the innervation patterns for the popliteus muscle and can be used as reference for its planning, development and execution surgical neurotomy techniques for a reduction of the muscular mass as well as for the treatment of spastisty in the posterior aspect of the leg also the presence of these findings can be considered during the planning of the surgical approach of the medial and lateral tibial plateau in the posterior aspect of the knee.

Figure 2 :
Figure 2: A. Right knee posterior view, first branch to popliteus muscle.

Table 1 .
Morphometric characteristics of popliteus tendon in different population groups.