FEMORAL FIT IN ABG-II HIP STEMS: INFLUENCE ON CLINICAL OUTCOME AND BONE REMODELLING.

Abstract

Introduction: In uncemented total hip arthroplasty (THA) stem sizing and stem insertion affect the fit and fill of the prosthesis in the medullary canal. This study investigates how tightness of the stem fit influences bone remodelling and if there is a correlation between radiological and clinical Results: Methods: In a retrospective study a consecutive series of 64 patients following uncemented THA with a proximally coated anatomic stem (ABG-II, Stryker) was followed-up for 5 years using the Merle d’Aubigne (MdA) clinical score. Radiographic analysis of bone remodelling features per Gruen zone (R1 to R7) was performed on AP and lateral x-rays at 5 years. Femoral fit was measured at three levels (proximal, mid-stem, distal) on the direct postop x-ray using the femoral fit ratio (f) of Kim and Kim (tight femoral fit: f≥0.8, non-tight: f< 0.8). The medullary canals were categorised according to Noble (normal, stove pipe, champagne flute). Bone remodelling was compared to literature values of the ABG-I stem and correlated to clinical findings.

Results: The MdA improved from 9.6 pre-op to 17.1 at 5 years with no difference between tight and non-tight implants. Lateral thigh pain (LTP) occurred in 10/64 cases (3 requiring medication). Patients with LTP had significantly lower proximal (0.75 vs 0.80) and distal fit & fill (0.72 vs 0.79. LTP was equally frequent with a normal or varus position.

Proximal bone resorption occurred in 27% (R1) or 34% (R7) which is lower than the values reported for the ABG-I stem (R1: 48%, R7: 45%). Bone resorption was significantly higher with tight than non-tight mid-stem fit (69% vs 27%, p=0.04). The same trend was true for tight distal fit (56% vs 37%).

Cancellous densifications were frequent at mid-stem level (R2: 83%, R6:88%) but much less distally (R3: 44%, R5:25%). No influence of fit & fill was measured.

Cortical densifications were noted in 16% (ABG-I 15%) overall with a higher proportion measured for tight distal fit (25%) than loose distal fit (6%, p=0.07). A similar observation was made for cortical thickening (11% overall, tight:non-tight=16%:6%). Pedestal formation (17% overall) was more likely with a non-tight proximal fit (23% vs 12%) and mid-stem fit (20% vs 8%)

A proximal tight fit was achieved more frequently with normal (55%) and stovepipe femora (50%) than champagne flute femora which had the highest proportion of tight distal fit (85%).

Discussion: At 5 years femoral implant fit influenced bone remodelling reactions which are in agreement with the design philosophy of proximally press-fitting anatomic stems. However, implant fit could not be correlated to clinical outcome yet. This may require a longer follow-up.

As LTP occurred with non-tight fit it seems that elastic mismatch is not the main cause.

Less proximal bone resorption and less distal densifications confirm the design changes from ABG-I to ABG-II.