Laboratory data indicate the hip capsular ligaments prevent excessive range of motion, may
protect the joint against adverse edge loading and contribute to synovial fluid replenishment
at the cartilage surfaces of the joint. However, their repair after joint preserving or
arthroplasty surgery is not routine. In order to restore their biomechanical function after hip
surgery, the positions of the hip at which the ligaments engage, together with their tensions
when they engage is required. Nine cadaveric left hips without pathology were skeletonised
except for the hip joint capsule and mounted in a six-degrees-of-freedom testing rig. A 5Nm
torque was applied to all rotational degrees-of-freedom separately to quantify the passive
restraint envelope throughout the available range of motion with the hip functionally loaded.
The capsular ligaments allowed the hip to internally/externally rotate with a large range of
un-resisted rotation (up to 50±10°) in mid-flexion and mid-ab/adduction but this was reduced
towards the limits of flexion/extension and ab/adduction such that there was a near-zero
slack region in some positions (p<0.014). The slack region was not symmetrical; the midslack
point was found with internal rotation in extension and external rotation in flexion
(p<0.001). The torsional stiffness of the capsular ligamentous restraint averaged
0.8±0.3Nm/° and was greater in positions where there were large slack regions. These data
provide a target for restoration of normal capsular ligament tensions after joint preserving hip
surgery. Ligament repair is technically demanding, particularly for arthroscopic procedures,
but failing to restore their function may increase the risk of osteoarthritic degeneration.