Muscular Architecture of the Posterior Knee and the Basic Science Implications
Wagner, Russell MD
Reeves, Rustin PhD
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Introduction: Musculoskeletal modelling plays an integral role in estimating clinically relevant muscle and joint contact forces which rely upon cadaveric experimentation. The muscular architecture of the posterior knee is often overlooked; yet can play an important role in knee mechanics and balancing. In this sense, the contribution of the popliteus muscle to a well-functioning total knee arthroplasty is often debated in the literature and requires further clarification. Furthermore, prior literature involved smaller sample sizes and did not look for differences in muscle architecture between males and females. Methods: 12 embalmed cadaver specimens were dissected to reveal the origin and insertion of the gastrocnemius, semimembranosus, and popliteus muscles. The orientation of these muscles and the breadth of the insertion of the popliteus muscle were recorded in relation to the long axis of the tibia using a goniometer. Muscle volume was assessed via water displacement can and graduated cylinder. Muscle fiber length and pennation angle were determined under a dissecting scope using a ruler and goniometer. Fiber length was determined via dissection and measurement from 3 separate areas and averaged for each muscle. Using these data, physiological cross sectional area (PCSA) was calculated by multiplying each muscle’s volume by the cos of the pennation angle and then dividing by it’s fiber length. Results were initially analyzed using descriptive statistics. Comparison between groups was performed via ANOVA with a post hoc Tukey test for multiple comparisons. Results: Mean muscle volumes for females: popliteus 12.6 ml, gastrocnemius 108.1 ml, and semimembranosus 81.9 ml (n=7). Mean muscle volumes for males: popliteus 20.5 ml, gastrocnemius 195.6 ml, and semimembranosus 174.3 ml (n=5). Significant differences between males and females were found in all three volumes (p=.001, p=.002, and p=.010 respectively). Significant differences between males and females were also found in PCSA for the popliteus and semimembranosus muscles (p=.008, p=.003 respectively). There were no significant differences found between males and females in fiber length, overall muscle length (excludes tendon), or orientation (Table 1,2,3). The mean orientations of the popliteus, medial gastrocnemius, and lateral gastrocnemius with respect to the long axis of the tibia were 145.3 degrees, 163.8 degrees, and 162.4 degrees respectively. The tibial attachment site of the popliteus muscle spanned between 38.3 degrees and 25.5 degrees in relation to the long axis of the tibia. Several data points were unable to be adequately collected due to incidents occurring during dissection (represented by the letter x in the tables). Results were compared to prior literature when possible and were found to be similar. Discussion and Conclusion: Currently, few musculoskeletal models include the popliteus muscle for kinematic and kinetic studies of the knee. The role of the popliteus muscle in knee mechanics and balancing should not be underestimated and merits inclusion into computational knee models and joint simulations. The ratio of popliteus PCSA to semimembranosus PCSA was 1:2.35 in females and 1:3.03 in males with an overall ratio of 1:2.69 irrespective of sex. These ratios, combined with the orientation of the popliteus, infer that the muscle plays a significant role in force generation across the knee joint. These findings also illustrate the need for subject-specific PCSA to be calculated for more reliable modelling due to the wide degree of muscular variation being present.