Healthy Hip Joints Have Different Macroscopic and Microscopic Capsular Nerve Architecture Compared With Hips With Osteoarthritis, Femoroacetabular Impingement Syndrome, and Developmental Dysplasia of the Hip: A Systematic Review

PURPOSE: To perform a systematic review to identify macroscopic and microscopic patterns and differences in hip capsule innervation between normal hips and hips with osteoarthritis (OA), femoroacetabular impingement (FAI) syndrome, and developmental dysplasia of the hip (DDH). METHODS: A systematic review was performed using Preferred Reporting Items for Systematic reviews and Meta-Analyses guidelines. Multiple databases were searched for both clinical and basic science laboratory studies on hip capsule innervation. Non-innervation capsule and non-human animal studies were excluded. Macroscopic and microscopic differences in capsular innervation between normal hips, and hips with OA, FAI, and DDH were analyzed. Methodological quality assessment of all studies included in this review was completed using the Methodological Index for Non-randomized Studies. RESULTS: Ten articles were analyzed (263 specimens; 211 patients, 52 cadavers; mean Methodological Index for Non-randomized Studies 10/16). The hip capsule is innervated by the sciatic and superior gluteal nerves posterosuperiorly, nerve to quadratus femoris and inferior gluteal nerve posteroinferiorly, and femoral and obturator nerves anteriorly. The anterior-superior capsule between 1:00 and 2:30 o’clock on a right hip is a safe internervous zone. The superolateral capsule has the greatest density of mechanoreceptors and sensory fibers. OA is associated with a greater expression of nerve fibers compared with normal hips but does not correlate with pain or disability. No significant differences were found in nerve fiber expression among patients with DDH, FAI, or normal hips. A negative correlation is seen with aging and pain fiber expression. CONCLUSIONS: The hip capsule has a complex macroscopic and microscopic innervation pattern with varying nerve fiber expression from at least 6 separate peripheral nerves. OA is associated with a greater expression of nerve fibers, although nerve fiber expression does not correlate with painful pathology. LEVEL OF EVIDENCE: IV, Systematic review of level I-IV studies.