The posterior capsule is variously incised and excised during total hip replacement (THR). There is no consensus on the direction of the capsulotomy and the need to repair the posterior capsule. The objective of this study was to determine the orientation of the collagen fibres and nerves in the posterior hip capsule in patients undergoing THR.

Specimens from five patients with osteoarthrosis of the hip (with no fixed deformity) were obtained and fixed in 10% neutral buffered formalin. Sutures were placed to mark the head and trochanteric end before excising. A standard posterior approach was used. The samples were examined and reported by a pathologist. Samples were processed overnight in a VIP5 automatic tissue processor and embedded in paraffin wax, preserving the location of the suture sites on embedding. Sections were cut at 5 Ïm and routinely stained with haematoxylin and eosin. The van Gieson stain was used for collagen fibres. Nerve fibres were highlighted using immunohistochemistry for S100 protein and blood vessels using an antibody to CD34.

The collagen bundles seen were predominantly parallel to the axis of the specimen. Dispersed within the collagen bundles were small vascular leashes that were parallel with the collagen fibres. The S100 staining revealed that these were neurovascular leashes, with small nerves running alongside the vessels and the collagen. Nerves that separate from the vessels were likely to serve proprioceptive and nociceptive functions.

The direction of the capsulotomy during THR by posterior approach has been traditionally perpendicular to the direction of the capsular fibres. However, if possible, capsulotomy along the orientation of the collagen fibres may be advantageous. As this study demonstrates, it will result in less damage to the capsular collagen fibres, blood vessels and nerves resulting in better capsular repair and healing, and better conservation of pro-prioceptive and nociceptive functions.