Incorporation and Remodeling of Structural Allografts in Acetabular Reconstruction

Abstract

Background:Total hip arthroplasty (THA) is frequently accompanied by acetabular bone loss, which constitutes a major challenge in revision procedures. Structural allografts can be implanted to restore a stable osseous foundation for the acetabular prosthesis. As previous studies were limited to clinical data or included very few cases, the extent to which the graft bone is incorporated over time has remained unclear.Methods:Thirteen acetabula were retrieved post mortem, and the incorporation properties of the bone allografts were analyzed using a hierarchical approach of imaging techniques including contact radiography, high-resolution peripheral quantitative computed tomography (HR-pQCT), histological analysis of undecalcified specimens, and quantitative backscattered electron imaging (qBEI). The distance between the current allograft bone and host bone borders (i.e., current overlap) as well as the distance between the original allograft bone and host bone borders (i.e., total ingrowth) were assessed.Results:In 10 of 13 cases, the complete interface (100%) was characterized by direct contact and additional overlap of the allograft bone and host bone, while the remaining 3 cases demonstrated direct contact along 25% to 80% of the interface. The allograft bone showed an intact trabecular structure and significantly higher mineralization compared with the host bone. The mean current overlap (and standard deviation) was 2.3 ± 1.0 mm, with a maximum of 5.3 ± 2.4 mm. Importantly, the total ingrowth reached much further, to a mean of 7.2 ± 2.3 mm (maximum, 10.5 ± 4.0 mm). Neither the time that the allograft was in situ nor the degree of contact between the host and allograft bone correlated with the current overlap and the time in situ did not correlate with total ingrowth.Conclusions:This study showed bone remodeling with subsequent interconnection of the host and allograft bone along the majority of the interface, leading to adequate incorporation of the allograft. The lack of complete incorporation of the graft did not lead to graft collapse up to 22 years after revision surgery.Clinical Relevance:Our study provides the first systematic multiscale evaluation of successfully implanted structural allografts and forms the scientific basis for their clinical use in revision THA.

Cellular Mechanisms Responsible for Success and Failure of Bone Substitute Materials

“…More specifically, a revascularization of the graft was followed by osteoclastic resorption and new bone formation on the graft. In structural allografts, successful incorporation and remodeling was also detected [ 9 ], however, this was most likely limited to the area of direct contact between the host bone and the allograft bone [ 30 ]. In general, the difference in the remodeling of chips and structural allografts may be explained by the different surgical techniques (i.e., impaction grafting vs. maintained structure).…”

“…Regarding the time course of allograft incorporation, it was found that allograft chips had incorporated within the first 12 weeks after implantation [ 32 ]. In structural allografts, long-term follow up observations revealed no time-dependent increase in the incorporation between four years and 22 years after implantation [ 9 ].…”

“…The main advantage of this technique is that screws and implants, such as tantalum augmentations, can also be visualized in order to estimate the full area of incorporation ( Figure 3 ). These mechanical barriers result in an absent local remodeling with insufficient incorporation of the graft around them [ 9 ]. On a higher magnified scale, the osteoconductive ability of allografts can be demonstrated by various other techniques.…”

“…Furthermore, while superficial areas of structural allografts are often completely remodeled, the center of the allograft remains unremodeled. Therefore, despite the very good clinical outcome of these allografts, as well as the impression of a completely remodeled graft in conventional imaging [ 49 , 50 , 51 , 52 ], there are indications that the incorporation might be less pronounced than previously expected (at least in humans) [ 9 ]. Also, synthetic materials showed incomplete remodeling with detectable remnants in human studies [ 1 ], although various animal studies indicated a complete remodeling [ 53 ].…”

“…Several histological and micro-morphological studies from retrieved specimens have proven the successful incorporation of bone substitutes such as allografts or synthetic grafts [ 1 , 7 , 8 ]. For allografts, bone remodeling with the subsequent interconnection of the host bone and the graft bone was found in the majority of the interface, leading to the progressive incorporation of the bone graft [ 9 ]. Furthermore, synthetic materials such as beta-tricalcium phosphate (β-TCP) were also found to induce bone formation and promote bone repair [ 1 ].…”