Purpose of the study  

Most imageless navigation systems for computer-assisted (CAS) total hip arthroplasty (THA) aim at orientating the cup with regard to the anterior pelvic plane (APP). However, these systems have some limitations: 1) the adequate position is not well defined to prevent impingement and dislocation; 2) dynamic variation of the APP according to activities (sitting, climbing stairs, etc.) are not integrated in determining the adequate orientation; 3) intraoperative assessment of the APP is not reliable with conventional tools, requiring ultrasound or two-stage identification. To address these issues, we developed an imageless CAS system without using APP, based on a kinematic approach of the hip joint. This system does not use the APP as the reference plane to orient the cup. The systemhelps the surgeon to orient the cup in relation to the cone describing the hip joint range of motion. The purpose of this study was to detail the technique and to analyze preliminary results.

Materials and methods  

Twenty-four primary cementless THAs were implanted using CAS Pleos™ with optoelectronic tracking system (18 women, 6 men; mean age, 67 ± 7.8 years, age range, 54–83 years) because of primary osteoarthritis. Two optoelectronic sensors are fixed percutaneously on the pelvis and the distal femur. The acetabulum is prepared first, followed by the femur using reamers and broaches of increasing size. The last broach placed in the femur was equipped with a large head adapted to the newly prepared acetabulum. The range of hip motion is recorded to determine the maximal range of motion cone. The acetabular cup is thus positioned so that the prosthesis range of motion totally covers the maximal range of motion of the hip joint.

Results  

The Postel-Merle-d’Aubigné score improved from 8.1 ± 3.2 (range, 3–13) preoperatively to 17.1 ± 0.8 (range, 16–18) at last follow-up. There were no complaints of patients about the sensor insertion and no cases of hematoma or fracture. Operative time was 35–40 min longer for the first four cases and was progressively reduced by 15–20 min for the last four cases. Mean leg length discrepancy was 5.6 ± 7.5 mm (range, 0–25 mm) before implantation and 0.6 ± 3 mm (range, −5 to 10 mm) at last follow-up, eighteen were equal. Mean frontal cup inclination was 47 ± 7° (range, 38–60°). After CT-scan measurement, mean anteversion of the femoral implant was 16.8 ± 9.2° (0–31°). The mean cup anteversion was 25.2 ± 9.2° (range, 8–40°) for “anatomical anteversion”. Only 10 of the 24 cups were orientated inside the Lewinnek safe zone (there was only one dislocation that was traumatic with a cup orientated in the safe zone).

Conclusion  

This method can be used in routine procedures without lengthening operative time significantly. It safely controls leg length and helps position the cup. This study demonstrates that there is no ideal position for the cup that can be used for all patients. Because of the wide range of inclination and anteversion figures, 58% of the cups were outside the safe zone recommended by Lewinnek.