summary Hip Arthrodesis is the fusion of the hip joint most commonly performed for management of advanced hip arthritis in a select group of patients. Common indications include salvage option for a failed THA, and for young active laborers with painful unilateral ankylosis after infection or trauma. Ideal hip fusion position is 20-35° of flexion, 0-5° adduction, 5-10° external rotation. Biomechanics Pathomechanics reduces efficiency of gait by ~50% increases pelvic rotation of contralateral hip increases stress at adjacent joints Biochemistry increases oxygen consumption requires 30% more energy expenditure for ambulation Treatment Primary hip arthrodesis indications salvage for failed THA (most common) young active laborers with painful unilateral ankylosis after infection or trauma neuropathic arthropathy tumor resection contraindications active infection severe limb-length discrepancy greater than 2.0 cm. bilateral hip arthritis adjacent joint degenerative changes lumbar spine contralateral hip ipsilateral knee severe osteoporosis degenerative changes in lumbar spine contralateral THA increased failure rate (40%) in THA when there is a contralateral hip arthrodesis Conversion of fusion to THA indications severely debilitating back pain (most common) severe ipsilateral knee pain with instability severe contralateral hip pain techniques obtain preoperative EMG to assess the status of the gluteus medius a constrained acetabular component is required if the abductor complex is nonfunctional outcomes clinical outcome is dependent on abductor complex function the presence of hip abductor complex weakness or dysfunction requires prolonged rehabilitation severe lurching gait may develop Techniques Hip arthrodesis goals achieve apposition of arthrodesis surfaces, obtain rigid internal fixation and promote early mobilization optimal position optimal positioning for function and limited effect on adjacent joints 20-35° of flexion 0°-5° adduction 5-10° external rotation avoid abduction as it creates pelvic obliquity and increased back pain approach lateral approach with trochanteric osteotomy is preferred important to preserve the abductor complex avoid injury to the superior gluteal nerve anterior approach to hip is also popular instrumentation cobra plating Complications Low back pain can be improved by taking down hip arthrodesis, but overall improvement depends on abductor function Ipsilateral knee degeneration and laxity Contralateral hip degeneration Prognosis Provides pain relief and reasonable clinical results in most patients Success may be limited by adjacent joint degeneration in 60% of patients lumbar spine, ipsilateral knee or contralateral hip may be affected low back pain and arthritic ipsilateral knee pain are the most common symptoms may start within 25 years of hip arthrodesis