Do Patients Who Undergo Conversion of Failed Osteoarticular Allografts to Megaprostheses Have Worse Outcomes Than Patients Treated With Primary Megaprostheses at Initial Resection?

Background Osteoarticular allografts are used for knee reconstruction after tumor resection, but the number of these types of reconstruction has declined because of frequent complications and perceived better results with tumor endoprostheses. Failed osteoarticular allografts can be salvaged with megaprosthesis conversion, but it remains unclear whether the outcomes of the patients who undergo conversion differ from those of patients who have primary megaprosthesis reconstructions at the time of tumor resection. Also, there is lack of evidence on the comparison of implant survival. This study aimed to fill the gaps in our current knowledge about conversion megaprosthesis to inform patients and guide treatment accordingly. Questions/purposes (1) Did the survivorship free from revision or amputation differ between patients who initially received an osteoarticular allograft and underwent a conversion to a megaprosthesis and patients who underwent primary megaprosthesis reconstruction? (2) Did the groups differ in terms of Musculoskeletal Tumor Society-93 (MSTS-93) scores, knee flexion angle, and extensor lag? Methods Between January 1997 and December 2023, we treated 276 patients with megaprosthesis for bone sarcoma of the distal femur and proximal tibia, including patients who had a failed osteoarticular allograft in the distal femur or proximal tibia and were subsequently converted to megaprosthesis as well as patients who initially underwent megaprosthesis reconstruction. At our institution, initial patient selection for osteoarticular allograft reconstruction is influenced by several factors. Patients who were younger, had fewer comorbidities, and who would be more compliant with the follow-up protocol usually underwent osteoarticular allograft reconstruction for the initial resection procedure. All patients were followed up for at least 2 years after megaprosthesis reconstruction or until reoperation or death (mean ± SD follow-up time 10 ± 8 years). The patient cohort in the institutional database had 29 patients with conversion before the exclusion criteria were applied. Seven percent (2 of 29) of the patients underwent conversion with different implant systems and 14% (4 of 29) of the patients had insufficient follow-up. Seventy-nine percent (23 of 29) of the patients were eligible for final analysis. In the primary group, there were 247 patients before the exclusion criteria were applied. Twenty-one percent (52 of 247) of the patients underwent primary reconstruction with different implant systems, 6% (14 of 247) received uncemented stems, 2% (6 of 247) underwent extraarticular resection, and 1% (2 of 247) of the patients initially underwent allograft-prosthetic composite reconstruction. After the exclusion criteria were applied, 70% (173 of 247) of the patients were eligible. Of those patients, 20% (49 of 247) had insufficient follow-up. Fifty percent (124 of 247) of the patients were eligible for the final analysis. After 1:2 nearest-neighbor propensity score matching, 23 patients who underwent a conversion to endoprosthesis were compared with 46 patients who had a primary megaprosthesis implanted at the time of the tumor resection. Groups were matched for age, gender, BMI, diagnosis, tumor location, resection ratio, stem length, and stem diameter. After matching, osteosarcoma was the most common diagnosis for the conversion and primary groups (65% versus 70%, standardized mean difference SMD -0.09). Distal femur was the most common tumor location for the conversion and primary groups (70% versus 70%, SMD 0.00). We compared survivorship free from revision and survivorship free from amputation between patients with conversion and primary megaprostheses using competing risk analysis. Tumor recurrence and death were assessed as competing events for competing risk analysis. We then compared functional outcomes, including the MSTS-93 score, knee flexion angle, and extensor lag with the Welch t-test. Additionally, we performed a noninferiority test to evaluate whether patients with conversion had functional outcomes (MSTS-93 score, knee flexion angle, and extensor lag) not worse than the primary group with a 15% margin. In comparisons, MSTS-93 scores and knee flexion angles were regarded as clinically inferior in lower values, whereas extensor lag was regarded as clinically inferior in higher values. Results Survivorship free from revision did not differ between the conversion and primary groups at 5 years (79% 95% confidence interval (CI) 60% to 98% versus 75% 95% CI 61% to 90%) and at 15 years (37% 95% CI 6% to 67% versus 49% 95% CI 25% to 73%; p = 0.61). Survivorship free from amputations did not differ between the conversion and primary groups at 15 years (100% 95% CI 100% to 100% versus 92% 95% CI 81% to 100%; p = 0.29). Likewise, the conversion and primary groups did not differ in terms of MSTS-93 scores at mean ± SD 11 ± 8 years (23 ± 4 versus 25 ± 5, mean difference 1 95% CI -1 to 4; p = 0.17). The conversion and primary groups did not differ in terms of knee flexion angles at 11 ± 8 years (94° ± 24° versus 104° ± 19°; p = 0.17). The conversion and primary groups did not differ in terms of extensor lag at 11 ± 8 years (3° ± 3° versus 6° ± 9°; p = 0.21). The conversion and primary groups differed in terms of extensor lag in proximal tibia replacements (5° ± 4° versus 13° ± 12°; p = 0.01). Noninferiority was demonstrated for MSTS-93 scores and extensor lag for the conversion group. Conclusion A conversion megaprosthesis yields outcomes no different than a primary megaprosthesis in terms implant survival, limb preservation, and functional outcomes. When limb salvage is feasible, a conversion megaprosthesis represents a reliable salvage option for failed osteoarticular allograft reconstruction. Our findings show that osteoarticular allografts around the knee continue to have a selective role as a time-buying strategy for younger patients with bone tumors without compromising future survival and outcomes. Level of Evidence Level III, therapeutic study.