CORR Insights®: How Often Do Acetabular Erosions Occur After Bipolar Hip Endoprostheses in Patients With Malignant Tumors and Are Erosions Associated With Outcomes Scores?

Where Are We Now?

Recent literature suggests that active patients with femoral neck fractures may benefit from a THA over a hemiarthroplasty, since THA may result in higher functional scores and a lower risk of revision to treat secondary acetabular wear [3, 10]. In older patients, however, the decision is more controversial, as hemiarthroplasty has reduced dislocation rates and is a less complex surgery to perform compared to THA. But even in elderly patients, these advantages should be weighed against the risk that hemiarthroplasty may still undergo conversion to THA because of groin pain caused by progressive loss of acetabular cartilage. A retrospective study of 686 patients treated for displaced femoral neck fractures found a low risk of subsequent revision to THA (2.5% at an average of 1.9 years after index procedure), consistent with other studies [5].

While many studies have compared THA to hemiarthroplasty in patients with femoral neck fractures, no studies to my knowledge have compared THA to hemiarthroplasty for endoprosthetic reconstruction of the proximal femur in patients with malignant or aggressive benign tumors. Furthermore, oncologic reconstructions are more difficult to compare because the patients, underlying diseases, and soft-tissue envelopes after resection are so variable.

In a study on 65 patients who underwent proximal femur tumor resection and bipolar proximal femur replacement, Drexler and colleagues [4] found that 4.6% underwent later conversion to THA to treat groin pain at a mean followup of 9.1 years. The current study by Houdek and colleagues [6] noted a similarly low risk of conversion (4% at a mean followup of 79 months), despite identifying acetabular erosion in 35% of their patients (mean change in acetabular cartilage thickness: 1.2 mm). Musculoskeletal Tumor Society (MSTS) scores did not correlate with acetabular erosion, which is consistent with the discrepancy between the conversion rate and radiographic evidence of acetabular wear.

Stevenson and colleagues [9] reviewed 100 proximal femur replacements with a mean followup of 3.6 years and found no radiographic evidence of wear in 84% of the patients, although they did note that those with evidence of wear did have longer radiologic followup.

Where Do We Need To Go?

It is intuitive that acetabular erosion correlates with followup length, as noted in the current study [6]. What is less obvious, however, is the clinical relevance of these changes in acetabular cartilage thickness [7]. There are certain patients with acetabular erosion and groin pain who might benefit from conversion to a THA, even if the number of these patients is low. But questions remain: What degree of degenerative acetabular changes are relevant? Do oncology patients benefit from this procedure?

With the well-documented success of bipolar endoprosthetic replacement following tumor resection of the proximal femur [2], advocating for THA, a more-complex operation with additional complications, should be done only with caution. Furthermore, given the large proportion of patients with these diagnoses who die within a few years of the index reconstruction (70% in the current study [6]), progressive acetabular erosion of unclear clinical importance may be a purely academic discussion.

Most of the data on functional outcomes in patients with oncologic diagnoses related to proximal femur replacements and acetabular erosion utilize the MSTS score. Meanwhile, the Harris Hip Score is most frequently utilized to assess the clinical outcome of total hip replacement, and other validated quality-of-life measuring tools are available such as the Toronto Extremity Salvage Score, WOMAC questionnaire, and Patient-Reported Outcomes Measurement Information System measures. Perhaps the lack of statistical difference between acetabular erosion and function is not truly a reflection of the clinical situation, but rather the lack of sensitivity of the instruments. Similarly, there are several methods utilized to measure and grade acetabular erosion in common use [1, 7]. Considering the soft-tissue alterations that accompany oncologic resections, and the associated changes in joint reaction forces, the ideal outcome measure or system for oncology patients remains unknown.

How Do We Get There?

It is unlikely that most patients with metastatic disease will benefit from conversion to THA to treat groin pain caused by acetabular wear. However, as patients with primary sarcoma live longer, acetabular cartilage erosions may indeed cause symptoms. Coordinated long-term followup studies that separate patients with primary sarcomas from those with metastatic disease could address the unanswered question of who may benefit from a proximal femur replacement with acetabular reconstruction versus a hemiarthroplasty.

It is incumbent on the orthopaedic oncology community to standardize patient-reported outcome measures, assessment tools, and grading systems recognizing the unique needs of our patients. The number of outcome instruments available and being reported has expanded. Given the limited number of proximal femur replacements performed each year, caution is needed to ensure we do not dilute the data pool.

Finally, as is the case with most joint replacement surgeries, a national registry would be a great asset to investigate proximal femur reconstruction. Outcomes over time for orthopaedic oncology implants utilizing a registry has been recognized as a priority among members of the MSTS [8]. The MSTS has recently developed a Committee for Registry Development that is actively coordinating with the American Academy of Orthopaedic Surgeons to design and implement an MSTS Registry. Perhaps these collective efforts will allow us to definitively determine the correct operation for our patients.