During Direct Anterior Approach Total Hip Arthroplasty (DATHA), the PT and the CT are released to elevate the proximal femur. While the PT is classified as a short external rotator tendon, it is crucial to retain the obturator externus tendon (OET) during surgery to prevent hip dislocation. [24, 30, 31] Cutting the OET can lead to hip instability, which can affect the surgical procedure and the obtained results. This study specifically focused on the impact of the PT and CT on hip joint stability, with all experiments and measurements conducted in the presence of the OET.
The relationship between the PT and the sciatic nerve is well-established. In DATHA, many hip surgeons consider the PT to play a role in maintaining hip stability. However, our previous study indicated that the elevation of the proximal femur is primarily limited by the CT. [24] Some scholars argue that the PT contributes more to stability than the CT. [18, 22, 23, 32] Our experiment also confirmed that the PT's role in preventing dislocation is dispensable, possibly due to its muscle and fiber direction. Any potential effects on the pelvis are likely to involve bottom-up control and have minimal impact.
The CT within the group of short external rotator muscles is the thickest and most versatile tendon, providing a firm grip on the greater trochanter and playing a crucial role in maintaining hip joint stability. [20, 21, 23, 32] Hip dislocation occurs when the femoral head's center of rotation does not align with the acetabulum's center of rotation, which can be indirectly determined by the position of the greater trochanter. [6, 13, 17, 33] The presence of the CT allows the greater trochanter to rotate during hip motion without changing its position, ensuring hip stability. The conjoined tendons also limit the backward motion of the femoral head to prevent posterior dislocation.
While other muscles and tendons contribute to hip stability, our experiments demonstrate the extensive and irreplaceable role of the CT in maintaining stability. Consequently, it is important to prioritize CT repair and preservation during Total Hip Arthroplasty (THA) to prevent dislocation.
However, our experiment has several limitations. The iliofemoral ligament, pubofemoral ligament, and sciatic ligament have been shown by many authors to be crucial for hip stability, and these stabilizing structures may have additional antagonistic and synergistic effects on the CT and PT. [25, 34, 35] Unfortunately, due to sample size limitations, we could not account for them in our study. The obturator externus tendon (OET) is also an important structure for maintaining hip joint stability. During hip dislocation, the OET may act as a lever and fulcrum, concentrating or dispersing forces generated by the GT space motion, thereby impacting hip stability. [23, 36, 37] However, for ease of comparison between the PT and CT, we did not include the OET in our study. Furthermore, passive hip dislocation in the supine position does not fully simulate real dislocation situations.
In conclusion, multiple factors, including age, gender, weight, adductor strength, acetabular tilt angle, acetabular and stem reversal, and offset, contribute to hip instability. [12, 13, 38] Analyzing these factors is critical for identifying the causes of dislocation following THA. Dislocation after hip surgery can be influenced by various factors, including bone structure, muscle and soft tissue integrity, and neuropathic factors. While our study focused on specific anatomical structures, future studies should aim for more comprehensive and refined investigations. [2, 6, 32, 33, 39] Performing successful primary hip surgery goes beyond basic surgical skills and requires an understanding of the potential triggers for hip dislocation following THA. Despite extensive research and debates on hip dislocation since the inception of joint surgery, conclusive findings remain elusive. Thus, there is still much to be fully understood regarding the influencing factors involved.