Intracapsular pressure and elasticity of the hip joint capsule in osteoarthritis

Abstract

Introduction The causes of pain in osteoarthrotic hip are unclear. Changes in hydrostatic intraarticular pressure have been suggested as one of the pain causing factors. The biomechanical properties of the hip joint capsule, such as elasticity, i.e. the compliance of the capsule, and its correlation to symptoms, severity of OA, joint effusion and intracapsular hydrostatic pressure to our knowledge have not previously been investigated. It is still unknown how prolonged synovitis/joint effusion, capsular tension and the severity of the OA affect the elasticity of the capsule. The purpose of this study was consequently to investigate the intracapsular pressure and elasticity of the joint capsule, i.e. capsular compliance and its relation to different stages of hip OA, and in relation to pain, symptoms and function. Patients and Methods We analyzed 31 unselected patients with hip osteoarthitis admitted for total hip replacement. The evaluation was based on estimation of radiographic osteoarthritis grade, HOOS hip score, sonographic measurements of capsular distance i.e. the distance between anterior capsule and femoral neck. Intracapsular pressure was measured peroperatively with Touhy needle inserted in the hip joint before excision of the capsule connected to a closed, non-volume consuming, sterile, monitoring set for invasive pressure connected to a pressure transducer. The hydrostatic intracapsular pressure with the hip in 45° of flexion was recorded, as well as in extension, extension-inward, and extension-outward rotation. Following these static pressure registrations we injected an increasing volume of saline, 1ml at a time, into the joint with 3 sec intervals between injections. The intracapsular pressure was continuously recorded until the intracapsular pressure reached 300mmHg which was the upper limit set to the pressure transduser. This recorded pressure/volume curve reflects the elasticity/compliance of the capsule, which could then be calculated.