Incorporation and Remodeling of Structural Allografts in Acetabular Reconstruction: Multiscale, Micro-Morphological Analysis of 13 Pelvic Explants

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.

Fig. 1

Timeline demonstrating the ages at the primary THA; allograft use during the first, second, third, or fourth revision arthroplasty; and analysis of the specimens at the time of death.

Fig. 2

Multiscale analysis of structural allograft incorporation. Red arrow = assumed location of the allograft. Fig. 2-A Anteroposterior radiograph made at the time of death. Fig. 2-B HR-pQCT scan indicates the 3-dimensional microstructural properties. Fig. 2-C Photograph of the cut section. Fig. 2-D Contact radiograph of the cut section. Fig. 2-E Microscopic overview of the ground section and determination of the interfaces. Blue line = host bone border, red line = allograft bone border, and green line = original allograft bone border. Fig. 2-F Polarized microscopy image indicating a cortical bone remnant from the allograft bone with an osteonal structure surrounded by cancellous vital host bone.

Fig. 3

Contact radiographs and the microscopic views of ground sections, showing successful osseoincorporation of structural allografts in Cases 4 (Fig. 3-A), 11 (Fig. 3-B), 2 (Fig. 3-C), and 8 (Fig. 3-D) (see Table I). The distance between the original allograft bone border (green line) and the host bone border (blue line) represents the total ingrowth, while the distance between the blue line and the red line (current allograft bone border) represents the current overlap.

Fig. 4

Histological analysis of ingrowth parameters from toluidine-blue-stained ground sections. Fig. 4-A Vital host bone (HB) was identified by the presence of viable osteocytes within the bone matrix and vital bone marrow cells. Fig. 4-B Dead allograft bone (AB) demonstrated black (air-filled) osteocyte lacunae. Figs. 4-C and 4-D Successful remodeling of the allograft bone surrounded by the host bone. Fig. 4-E Analysis of the current allograft bone-host bone interface. Blue line = host bone border, red line = allograft bone border, and asterisk = current overlap area. Figs. 4-F, 4-G, and 4-H Regression analysis of the percentage of the interface with direct contact between the host and allograft bone and the current overlap (Fig. 4-F), the time in situ and the current overlap (Fig. 4-G), and the time in situ and total ingrowth (Fig. 4-H).

Fig. 5

Results of qBEI analysis. The error bars on the graphs indicate the standard deviation (SD). Fig. 5-A Allograft bone revealed higher mineralization than the host bone, as expressed by the differences in the gray values. Red arrows = hypermineralized (micropetrotic) osteocyte lacunae in allograft bone. Fig. 5-B Mean calcium content. **P < 0.001. Fig. 5-C Calcium peak values, indicating the most frequent calcium content. **P < 0.001. Fig. 5-D Calcium width, indicating the mineralization heterogeneity. Fig. 5-E Overall bone mineral density distribution (BMDD), indicating a higher matrix mineralization in allograft bone. B.Ar = bone area. Fig. 5-F The number of osteocyte lacunae per bone area (N.Ot.Lc/B.Ar). *P < 0.05. Fig. 5-G Lacunar area (Lc.Ar). *P < 0.05.