Abstract
The human hip capsule is a heterogeneous structure contributing greatly to the stability of this joint. A posterior approach to the hip necessarily sacrifices the ischio-femoral ligament but the decision to release the ilio-femoral and pubo-femoral ligaments remains at the discretion of the surgeon. This mechanical study aims to demonstrate that these anterior capsular structures, when left intact, may limit the external rotational range of motion when the variables of femoral offset, leg length and neck version are adjusted at the time of surgery.
A dry bone pelvis-femur model was prepared and registered with the Stryker iNstride Hip Navigation software. A cemented 28 HDPE contemporary cup was inserted at 45° inclination with 20° of anteversion and a revision modular stem implanted in the femoral medullary canal. Artificial ilio-femoral and pubo-femoral ligaments were then prepared from plastinated rubber fabric and mounted in their anatomical positions. Using this model, a range of restoration body sizes was sequentially introduced to vary the offset. The rotational range of motion was then assessed. Repeat measurements were made using + 10mm length bodies across the same offset range. Finally, assessments of rotational range of motion were made using the 19mm body alone while varying neck lengths and degrees of version were trialled. All measurements of external rotation were taken in a position of 0° hip flexion and 0° abduction, as determined using the Stryker iNstride Hip Navigation System.
As femoral offset was increased using our model, there was a progressive loss of external rotation. This consistent restriction of external rotation was further accentuated when +10mm length bodies were trialled across the same range of offsets. When a standard 19mm restoration body was placed and a range of heads trialled, it was again found that increasing neck length consistently correlated with a reduction in external rotation. Varying the restoration neck version with a standard head, it was found that increasing retroversion correlated with an increase in the external rotational range of motion.
The findings of this mechanical study suggest a progressive limitation of hip external rotation with increasing femoral offset and leg length when the anterior capsular structures are intact. Such findings are of importance in pre-operative planning as they suggest that increases in these variables may significantly limit a patient’s range of external rotation unless the anterior hip capsule is released. Such considerations must of course be balanced against the potential to destabilise the hip if too extensive a soft tissue release is performed. The artificial model used in this study is intended to approximate the human hip and its ligaments. The absolute values for rotational range of motion measured using the Stryker hip navigation system are less significant than the overall trend which they suggest. A patient-based study is now planned to further test these findings.
Correspondence should be addressed to Associate Professor N. Susan Stott at Orthopaedic Department, Starship Children’s Hospital, Private Bag 92024, Auckland, New Zealand
