Abstract
Background
Implant stability and is an important factor for adequate bone remodelling and both are crucial in the long-term clinical survival of total hip arthroplasty (THA). Assessment of early bone remodelling on X-rays during the first 2 years post-operatively is mandatory when stepwise introduction of a new implant is performed. Regardless of fixation type (cemented or cementless), early acetabular component migration is usually the weakest link in THA, eventually leading to loosening. Over the past years, a shift towards uncemented cup designs has occurred. Besides the established hydroxyapatite (HA) coated uncemented cups which provide ongrowth of bone, new uncemented implant designs stimulating ingrowth of bone have increased in popularity. These cups initiate ingrowth of bone into the implant by their open metallic structure with peripheral pores, to obtain a mechanical interlock with the surrounding bone, thereby stabilising the prosthesis in an early stage after implantation. This retrospective study assessed bone remodelling, osseointegration and occurrence of radiolucency around a new ingrowth philosophy acetabular implant.
Methods
In a retrospectively, single centre cohort study all patients whom underwent primary THA with a Tritanium acetabular component in 2011 were included. Bone remodelling, osseointegration and occurrence of radiolucency were determined by two reviewers from X-ray images that were made at 6 weeks, 3–6-12 and 24 months post-operatively. Bone contact % was calculated based on the original Charnley and DeLee zones. According to Charnley and DeLee the outer surface of an acetabular cup is divided into 3 zones (1-2-3). For our analysis the original 3 zones were further divided into 2 producing 6 zones 1A to 3B. Each of these 6 zones were then further divided into 4 equal sections. We attributed 25 points per section in which complete bone contact without lucency was observed. If lucency was observed no points were attributed to the section. A fully osteointegrated cup in all 24 sections could therefore attain 600 points. The total of each section and zone was subsequently tallied and recalculated to produce the percentage of bone contact on a 1–100% score.
Results
In 2011 131 patients; 54 male and 76 female with average age of 60.83 (SD 12.42) and 60.57 (SD 12.11) year respectively underwent primary THA at our institution and all where included in our study cohort. Majority of this cohort underwent primary THA due to osteoarthrosis and most patients were classified ASO I (18%) or ASA II (65%). At two year clinical follow-up two revision were performed. One constituted a femur and acetabulum revision due to leg length difference and a snapping hip phenomenon. Complications included 3 dislocations (all treated policlinic), 4 deep infections (all treated with Genta PMMA beads with prosthesis in situ and healed) and 1 removal of hematoma. In another patient the femoral component was revised due to a peri-prosthetic fracture. Mean bone contact % values for all Charnley and DeLee zones combined were calculated and improved from 68,18% (SD 22,36) at 6 weeks to 73,61% SD (16,26) at 3 months to 84,21% (SD 19,02) at 6 months to 86,90% (SD 16,0) at 1 year to 92,19% (SD 12,74) at two year follow-up. When analysing the bone contact % per individual zone a remarkable difference was found for zones 2A-B. In contrast to zone 1A-B and 3A-B the initial bone contact % was clearly although not significantly lower until two year follow-up.
Conclusions
In this study, the bone apposition around Tritanium actebular component was retrospectively assessed until two year clinical. Our results show excellent bone apposition that continues to improve over time (at least until two year clinical follow-up) suggesting that the open trabecular Ti structure of the Tritanium has a positive effect on cup osseointegration. However, some recent reports have shown the development of reactive lines around cups with porous/trabecular metal surfaces, of which the meaning is still unclear. Our analysis indicated that especially acetabular zone 2A-B according to Charnley&DeLee needs time to establish a direct contact of the implant surface and the surrounding bone tissues. Perhaps this might be explained by reaming technique (underreaming vs line to line reaming) resulting in suboptimal seating of the cup. Therefore, careful follow-up of this new implant technology will remain necessary and continued in this study. We aim to improve cohort size and establish results at longer follow-up times. Furthermore we aim to correlate these results to RSA component migration analysis.
