Dual-Layer Spectral-Computed Tomography Enhances the Separability of Calcium-Based Implant Material from Bone: An Ex Vivo Quantitative Imaging Study.

Local treatment of bone loss with an injection of a resorbable, calcium-based implant material to replace bone has a long history of clinical use. The in vivo discrimination of changes in bone versus implant is challenging with standard computed tomography (CT). However, spectral-CT techniques enable the separation between tissues of similar densities but different chemical compositions. Dual-layer spectral-CT imaging and postprocessing analysis methods were applied to investigate the separability of AGN1 (a triphasic calcium-based implant) and bone after AGN1 injection in n = 10 male cadaveric femurs ex vivo. Using the area under the curve (AUC) from receiver-operating characteristic (ROC) analyses, the separability of AGN1 from bone was assessed for AGN1 (postoperatively) versus compact and versus femoral neck cancellous bone (both preoperatively). CT techniques included conventional Hounsfield (HU) and density-equivalent units (BMD, mg hydroxyapatite [HA]/cm3 ) and spectral-CT measures of effective atomic number (Zeff) and electron density (ED). The samples had a wide range of femoral neck BMD (55.66 to 241.71 mg HA/cm3 ). At the injection site average BMD, HU, Zeff, and ED increased from 69.5 mg HA/cm3 , 109 HU, 104.38 EDW, and 8.30 Zeff in the preoperative to 1233 mg HA/cm3 , 1741 HU, 181.27 EDW, and 13.55 Zeff in the postoperative CT scan, respectively. For compact bone at the femoral shaft the preoperative values were 1124.15 mg HA/cm3 , 1648 HU, 177 EDW, and 13.06 Zeff and were maintained postoperatively. Zeff showed substantially sharper distributions and significantly greater separability compared to ED, BMD, and HU (all p < 0.002, for both regions) with average AUCs for BMD, HU, ED, and Zeff of 0.670, 0.640, 0.645, and 0.753 for AGN1 versus compact and 0.996, 0.995, 0.994, and 0.998 for AGN1 versus femoral neck cancellous sites, respectively. Spectral-CT permits better discrimination of calcium-based implants like AGN1 from bone ex vivo. Our results warrant application of spectral-CT in patients undergoing procedures with similar implants. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

AGN1 implant material to treat bone loss: Resorbable implant forms normal bone with and without alendronate in a canine critical size humeral defect model.

BACKGROUND: Fractures secondary to osteoporosis, particularly those of the hip and spine, are a major public health concern with high social and economic costs. The Local Osteo-Enhancement Procedure (LOEP) is an approach intended to strengthen skeletal areas that are at the highest risk for fracture due to osteoporosis. LOEP involves the implantation of AGN1, a triphasic, calcium-based, osteoconductive material which is then resorbed and replaced by bone. Since alendronate is the most prescribed osteoporotic treatment, the purpose of this canine study is to determine if the newly formed bone has the same properties as normal bone and whether alendronate treatment impacts AGN1 resorption and replacement with bone. METHODS: Sixty skeletally mature male hounds (24-38 kg) were evenly divided between alendronate (0.2 mg/kg/day) and non-alendronate treatment groups. A critical-size core bone defect created in one proximal humerus was implanted with AGN1 while the contralateral non-operated humerus served as a paired control in each animal. Animals were sacrificed 13, 26, and 52 weeks post-operatively (10 per treatment per timepoint). The control and treatment site bone specimens from each animal were examined using radiographic, histomorphometric, and biomechanical techniques. Results between alendronate-treated and non-alendronate-treated animals were compared as groups. RESULTS: AGN1 implant material was consistently resorbed and replaced by bone in all animals. At 52 weeks, only minimal residual implant material could be detected (0.9 ± 2.3% non-alendronate group; 2.2 ± 3.1% alendronate group), and new bone filled the defects in both the non-alendronate and alendronate groups. At 13 and 26 weeks, microCT revealed the newly formed bone in the defects had significantly higher trabecular bone volume and number connectivity than control bone in both groups. Mechanical testing demonstrated that the new bone had ultimate compressive strength and modulus equivalent to control bone as early as 13 weeks post-surgery which was maintained to 52 weeks in both groups. CONCLUSIONS: In this canine critical-sized humeral core defect model, AGN1 was progressively replaced by normal bone as evaluated by all outcome measures. Concurrent alendronate therapy did not significantly impact AGN1 resorption or new bone formation. These results demonstrate that AGN1 can be used in conjunction with alendronate in non-osteoporotic animals. CLINICAL RELEVANCE: This study suggests that the AGN1 implant material demonstrates potential for local restoration of bone in critical-size core defects, and that the material is compatible with alendronate drug therapy. Further studies will be required to determine if these results apply to other osteoporosis medications.

In vitro injection of osteoporotic cadaveric femurs with a triphasic calcium-based implant confers immediate biomechanical integrity.

Current pharmaceutical therapies can reduce hip fractures by up to 50%, but compliance to treatment is low and therapies take up to 18 months to reduce risk. Thus, alternative or complementary approaches to reduce the risk of hip fracture are needed. The AGN1 local osteo-enhancement procedure (LOEP) is one such alternative approach, as it is designed to locally replace bone lost due to osteoporosis and provide immediate biomechanical benefit. This in vitro study evaluated the initial biomechanical impact of this treatment on human cadaveric femurs. We obtained 45 pairs of cadaveric femurs from women aged 77.8 ± 8.8 years. One femur of each pair was treated, while the contralateral femur served as an untreated control. Treatment included debridement, irrigation/suction, and injection of a triphasic calcium-based implant (AGN1). Mechanical testing of the femora was performed in a sideways fall configuration 24 h after treatment. Of the 45 pairs, 4 had normal, 16 osteopenic, and 25 osteoporotic BMD T-scores. Altogether, treatment increased failure load on average by 20.5% (p < 0.0001). In the subset of osteoporotic femurs, treatment increased failure load by 26% and work to failure by 45% (p < 0.01 for both). Treatment did not significantly affect stiffness in any group. These findings provide evidence that local delivery of the triphasic calcium-based implant in the proximal femur is technically feasible and provides immediate biomechanical benefit. Our results provide strong rationale for additional studies investigating the utility of this approach for reducing the risk of hip fracture. © 2019 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society.