Assessment using computed tomography of wear and osteolysis in uncemented cups

Abstract

Introduction: Uncemented acetabular cups are widely used since the early 1990’s, particularly on younger patients with osteoarthritis. Patients with these cups have a reported high risk for revision in the Swedish Hip Arthroplasty Register (SHAR) due to wear and periacetabular osteolysis (PAO). There are often no clinical symptoms from wear or osteolysis until cup loosening or total liner wear occurs. This might result in an extensive revision. Early detection of critical wear and osteolysis could prevent implant failure. Gold standard for postoperative assessment in THR is Diagnostic Radiographic images (DR). To detect wear and PAO, Computed Tomography (CT) can also be used. One concern with CT is the high radiation exposure. Other problems with CT of patients with metal implants are artifacts due to beam hardening effects, photon starvation, and scatter artifacts which obscure the area surrounding the implants.

Materials and methods: Study 1. We assessed 206 asymptomatic patients with a mean age of 54 years, with an uncemented cup with a median follow up of 10 years after surgery. Wear and PAO was measured using CT and Diagnostic Radiograpchic images (DR). Two independent observers evaluated the images twice, one month apart. Osteolysis was classified with a new classification system that was proposed for CT. PAO using DR was classified using the Saleh classification system. Study 2. We assessed the same cohort as study 1. The association between PAO and wear, age, gender, activity, BMI, cup type, cup age, positioning of the cup, and surface coating was investigated with a proportional oddsmodel. EQ5D, clinical assessment, pain from the hip and patient satisfaction was evaluated to verify that thesepatients were asymptomatic. Study 3. Patients from study 2 with more than 1/3 wear at 10 years were reassessed with CT 13 years after surgery.Wear and osteolysis were measured and the development of wear and PAO was analyzed. There were 46 patients included but 15 had either been revised, disappeared or died at 13 years postoperatively. Study 4. A swine pelvis with bilateral uncemented hip prostheses and with known “wear” and acetabular bonedefects, was examined using a third generation multidetector Dual Energy CT. Four different radiation levels were investigated. To maintain image quality even when radiation exposure levels were lowered we compared Monochromatic and Iterative reconstruction techniques to reduce artifacts and enhance the images.

Results: Study 1. On Diagnostic Radiographs, 192 cases showed no osteolysis and only 14 cases had osteolysis. With CT there were 184 cases showing small or large osteolysis and only 22 patients showing no osteolysis. A new validated classification system for PAO was proposed based on CT that was found easy to use on standard followup evaluation. Study 2. Wear was associated with an increased risk for periacetabular osteolysis (odds ratio 1.4). Male gender had, when isolated from other factors, some influence on PAO. There was no association between PAO to time from operation, patient age, UCLA Activity Score, liner thickness at time of operation, BMI, cup positioning or type of implant. A thin liner at time of operation was correlated to increased wear. Linear wear rate was 0,18 mm/year and 46 of 206 patients had large PAO. Study 3. For patients with more wear than average at 10 years postoperativly, the wear rate accelerated from 0.2 mm per year during the interval 0-10 years to 0.5 mm per year from 10 to 13 years postoperativly. Osteolysis also increased but high wear rate did not correlate to accelerated PAO in this cohort. Study 4. We found that we could successfully assess wear and PAO while lowering the effective dose from 1.5 mSv to 0.7 mSv. Our results indicate that prosthetic linear wear and periacetabular osteolysis probably could be assessed with clinically acceptable accuracy at a radiation exposure level on par with conventional radiographic examination.

Conclusions: Patients with uncemented cups without any symptoms of wear and periacetabular osteolysis should be followed with a CT in order to detect wear and osteolysis that might lead to cup loosening and/or head penetration when the liner is wearing out. With modern Dual Energy CT technique, radiation levels can be drastically reduced with well kept image quality.