Clinical Outcomes of Nonunions of Hindfoot and Ankle Fusions

Krause, Fabian MD; Younger, Alastair S.E. MBChB, ChM, MSc, FRCSC; Baumhauer, Judith F. MD, MPH; Daniels, Timothy R. MD, FRCSC; Glazebrook, Mark PhD, MD, FRCSC; Evangelista, Peter T. MD; Pinzur, Michael S. MD; Thevendran, Gowreeson MBChB, MFSEM(UK), FRCSEd(Tr&Orth); Donahue, Rafe M.J. PhD; DiGiovanni, Christopher W. MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.14.00872
Scientific Articles
Abstract

Background: While nonunion after foot and ankle fusion surgery has been associated with poor outcomes, we are not aware of any longitudinal study on this subject. Thus, we prospectively evaluated the impact of nonunion on clinical outcomes of foot and ankle fusions and identified potential risk factors for nonunion after these procedures.

Methods: Using data from a randomized clinical trial on recombinant human platelet-derived growth factor-BB (rhPDGF-BB; Augment Bone Graft, BioMimetic Therapeutics), union was defined either by assessment of computed tomography (CT) scans at 24 weeks by a reviewer blinded to the type of treatment or by the surgeon’s composite assessment of clinical and radiographic findings at 52 weeks and CT findings at 24 or 36 weeks. The nonunion and union groups (defined with each assessment) were then compared in terms of clinical outcome scores on the American Orthopaedic Foot & Ankle Society Ankle-Hindfoot Scale (AOFAS-AHS), Foot Function Index (FFI), and Short Form-12 (SF-12) as well as age, sex, body mass index (BMI), smoking status, diabetes status, work status, and arthrodesis site.

Results: Blinded CT assessment identified nonunion in 67 (18%) of 370 patients, and surgeon assessment found nonunion in 21 (5%) of 389 patients. Postoperatively, the nonunion group scored worse than the union group, regardless of the method used to define the nonunion, on the AOFAS-AHS and FFI, with mean differences of 10 and 12 points, respectively, when nonunion was determined by blinded CT assessment and 19 and 20 points when it was assessed by the surgeon. The nonunion group also had worse SF-12 Physical Component Summary scores. Differences between the union and nonunion groups were clinically meaningful for all outcome measures, regardless of the nonunion assessment method. The concept of an asymptomatic nonunion (i.e., imaging indicating nonunion but the patient doing well) was not supported. Patients with nonunion were more likely to be overweight, smokers, and not working.

Conclusions: This prospective longitudinal study demonstrated poorer functional outcomes in patients with a nonunion after foot and ankle fusion, regardless of whether the diagnosis of nonunion was based on CT only or on combined clinical, radiographic, and CT assessment.

Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Author Information

1Department of Orthopaedic Surgery, Inselspital, University of Bern, Bern, Switzerland

2Department of Orthopaedics, University of British Columbia, Vancouver, British Columbia, Canada

3BC’s Foot and Ankle Clinic, St. Paul’s Hospital, Vancouver, British Columbia, Canada

4Department of Orthopaedics, University of Rochester School of Medicine and Dentistry, Rochester, New York

5Division of Orthopaedic Surgery, St. Michael’s Hospital and University of Toronto, Toronto, Ontario, Canada

6Dalhousie University, Halifax, Nova Scotia, Canada

7Queen Elizabeth II Health Sciences Center, Halifax, Nova Scotia, Canada

8Division of Foot and Ankle Surgery, Department of Orthopaedic Surgery, Massachusetts General Hospital and Newton-Wellesley Hospital, Waltham, Massachusetts

9Department of Orthopaedic Surgery and Rehabilitation, Loyola University Health System, Maywood, Illinois

10Foot & Ankle Service, Department of Trauma & Orthopaedics, Tan Tock Seng Hospital, Singapore

11Wright Medical N.V., Franklin, Tennessee

E-mail address for A.S.E. Younger: asyounger@shaw.ca

Article Outline

Arthrodesis reduces pain and corrects osseous deformity associated with trauma, diabetes, rheumatoid arthritis, ligamentous instability, malalignment, and congenital deformities causing joint degeneration1-5. In the ankle and hindfoot, end-stage arthritis is often accompanied by marked pain, limited function, and impaired quality of life6.

Despite modern implants and advanced surgical techniques, the most common major complication of foot and ankle arthrodesis is nonunion. A nonunion is thought to cause a worse functional outcome, is commonly rated as an adverse event in the postoperative course7,8, and may ultimately lead to failure of the procedure and require a repeat arthrodesis9-12. Arthrodesis sites are frequently supplemented with autologous bone graft or bone substitutes to promote union7,13.

Nonunion is defined as incomplete osseous healing at ≥6 months following arthrodesis14. Until 2006, nonunions reported in studies were detected on radiographs. Computed tomography (CT) is more reliable, but no standardized test to confirm the diagnosis has been recommended to date13. Although there is no clear method of defining nonunion, some osseous bridging (≥25%) on CT has been found to result in a clinically important improvement in clinical outcome scores compared with the scores in patients without bridging15. Previous reports of nonunion following ankle arthrodesis have been based on retrospectively reviewed data, and nonunion has usually been determined through radiographic and/or clinical evaluation. In most retrospective studies, a CT scan was ordered only if the patient reported pain. If there is no clinical indication for investigation, a nonunion may not be identified at the time of follow-up and may be associated with an acceptable outcome. We are not aware of any prospective, longitudinal study demonstrating that a nonunion causes a worse clinical outcome compared with that of patients who obtain union.

Approximately 11% (range, 3% to 23%) of foot and ankle fusions result in nonunion7,12,16-18 as identified on radiographs, clinical evaluation, or both; this proportion is greater in patients with risk factors such as smoking, diabetes, and obesity19,20. A recent literature review demonstrated only fair evidence (in Level-II or III retrospective cohort or case-control studies) that smoking and diabetes are contributory systemic risk factors for nonunion20. Evidence that local factors (i.e., vascularity and local infection) are pertinent risk factors for nonunion is insufficient. Fair evidence has implicated soft-tissue injury as a predisposing factor for nonunion20.

A prior randomized, controlled clinical trial comparing graft materials in 397 patients undergoing foot and ankle fusion included periodic CT scans and radiographs21. This provided us with an opportunity to evaluate nonunions identified by prospectively collected data that may not have been otherwise identified in a retrospective study. In the current study, we employed the database from that trial to evaluate the impact of nonunion on clinical outcomes after foot and ankle arthrodesis and to identify risk factors for such nonunions. Nonunion was identified by assessment of 24-week CT scans by a reviewer blinded to the type of graft treatment and also separately by a combination of surgeon-performed CT, radiographic, and clinical assessments. Patient-reported clinical outcomes were compared between the union and nonunion groups for each of the 2 methodologies used to assess nonunion.

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Materials and Methods

Study Design

The original prospective, randomized, controlled, blinded, multicenter, clinical trial21 (registered at ClinicalTrials.gov: NCT00583375) was performed at 37 sites across the United States and Canada between April 2007 and January 2010. The U.S. Food and Drug Administration (FDA), Health Canada, institutional review boards, and research ethics boards approved the study. Patients meeting the inclusion criteria (Table I) were prospectively enrolled in accordance with FDA good clinical practice guidelines and provided informed consent.

In the initial trial21, 414 patients were randomized and treated with 1 of 2 graft materials (recombinant human platelet-derived growth factor-BB [rhPDGF-BB; Augment Bone Graft, BioMimetics] combined with beta-tricalcium phosphate [β-TCP] or autograft) for foot and ankle fusion (Fig. 1). Standardized operative techniques and postoperative protocols were used for all patients. The primary results of the comparison of the 2 treatments are reported separately21. Data compiled in the trial, including periodic radiographic assessment independent of symptoms, were used for this analysis.

The study protocol included 8 postoperative visits through 52 weeks. The patient’s clinical and functional outcomes were recorded and radiographs (weight-bearing anteroposterior, mortise, and lateral views of the ankle or anteroposterior, lateral, and axial heel views of the foot) were made during each visit. CT scans were performed at 9, 16, 24, and 36 weeks and were independently assessed by a fellowship-trained, board-certified, musculoskeletal radiologist blinded to the treatment received. Assessment of intraobserver reliability, based on repeat review of all 24-week CT scans 3 months later, showed a kappa of 0.87, 95% concordance, and 0.987 tetrachoric correlation.

Nonunion was defined by 2 methods. The first relied exclusively on blinded review of CT scans at 24 weeks by the independent radiologist. Union was defined as ≥25% osseous bridging across the articulation, as that had been demonstrated to be associated with clinically important improvement15. Second, nonunion was defined at 1 year by the treating surgeon on the basis of a composite evaluation of (1) CT scans at 24 or 36 weeks with union defined as ≥50% osseous bridging across the articulation13, (2) radiographs at 1 year with osseous union defined as osseous bridging across at least 3 of 4 predefined aspects (anterior, posterior, medial, and lateral) and disappearance of the joint space, and (3) clinical findings at 1 year, based on a global assessment at both the full joint level (i.e., considering the full complement of fusion sites) and the individual joint level (i.e., considering every joint independently), with therapeutic failure defined as any symptomatic nonunion or delayed union that required secondary therapeutic intervention. Radiographs and CT scans in isolation were not considered diagnostic and were only used in the context of clinical findings. Using this composite evaluation, the treating surgeon determined that there was union (Figs. 2 and 3), evidence of progressive healing, or nonunion (Figs. 4 and 5). Evidence of progressive healing and nonunion were both considered nonunion for this study. For both assessments of union, patients with concomitant arthrodeses of >1 joint (i.e., double or triple arthrodesis) were considered to have a nonunion if any of the joints were ununited.

For the current study, data collected in the original randomized controlled trial21 were reviewed to determine the clinical outcomes of patients with nonunion compared with those with successful arthrodesis, both with nonunion defined via blinded CT assessment and with it defined via the composite surgeon evaluation. We also assessed the impact of demographics and the fusion site on the development of nonunion. Patients were divided into “nonunion” and “union” groups for each assessment, and 4 groups were assessed: those with union on both assessments (i.e., blinded CT and surgeon assessments), those with union on surgeon assessment and nonunion on blinded CT assessment, those with nonunion on surgeon assessment and union on blinded CT assessment, and those with nonunion on both assessments. Functional assessment was performed with the American Orthopaedic Foot & Ankle Society Ankle-Hindfoot Scale (AOFAS-AHS)11, the Foot Function Index (FFI)22, and the Short Form-12 (SF-12)23 at the 1-year follow-up visit. Minimal clinically important differences are 8.9 points for the AOFAS-AHS24, 7 points for the FFI25, and 4.5 points for the SF-1226. The frequently used AOFAS-AHS is highly responsive and has shown acceptable criterion validity, but its reliability is unknown27.

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Statistical Analysis

The union and nonunion groups were compared to determine differences in age, sex, body mass index (BMI), smoking status, diabetes status, arthrodesis site (ankle and/or joints of the triple joint complex in isolation or combination), and functional outcome. Continuous variables (age and BMI) were compared using one-way analysis of variance. Categorical variables were compared using the Fisher exact test. The effect of union or nonunion status on outcome measure scores at 1 year was assessed after adjustment for confounders (diabetes, smoking, BMI, number of joints operated on, and job status) using analysis of covariance, with baseline scores as a covariate. A p value of ≤0.05 was considered significant. Results were reported as means with corresponding 95% confidence intervals (CIs).

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Results

Nonunion Diagnosed by Blinded CT Assessment

Complete CT data obtained at week 24 and functional outcome data obtained at week 52 were available for 370 patients (89%), 67 (18%) of whom demonstrated nonunion and 303 (82%) of whom demonstrated union, as assessed by the blinded CT reviewer.

All functional outcomes were better in the union group (Fig. 6). The mean AOFAS-AHS total score was 10 points higher (i.e., better) in the union group (80.0; CI = 78.4, 81.5) compared with the score in the nonunion group (69.6; CI = 64.8, 74.4). The mean FFI score was 12 points lower (i.e., better) in the union group (16.9; CI = 14.9, 18.9) compared with the score in the nonunion group (28.7; CI = 23.2, 34.2). The mean SF-12 physical component summary (PCS) score was 5 points higher (better) in the union group (44.2; CI = 43.1, 45.3) than in the nonunion group (39.4; CI = 37.0, 46.9).

When adjusted for confounders (diabetes, BMI, smoking, number of joints, and employment status), all functional outcome scores were significantly better in the union group than in the nonunion group (Table II).

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Nonunion Diagnosed by Surgeon Composite Evaluation of Clinical, CT, and Radiographic Findings

Complete data for evaluation by the surgeon at week 52 were available for 389 patients (94%) and 508 arthrodesis sites. Twenty-one patients (5%) were found to have a nonunion, and 368 (95%) demonstrated union.

All functional outcomes at 52 weeks were significantly better in the group that showed union based on surgeon assessment compared with the nonunion group (Fig. 7); however, the differences were larger than those seen in the groups that underwent blinded CT assessment. The mean AOFAS-AHS total score was 19 points higher in the union group (78.8; CI = 77.3, 80.3) compared with the nonunion group (59.8; CI = 49.5, 70.0). The mean FFI score was 20 points lower in the union group (18.2; CI = 16.3, 20.1) compared with the nonunion group (38.7; CI = 25.6, 51.8). The mean SF-12 PCS score was 6 points higher in the union group (43.6; CI = 42.6, 44.6) than in the nonunion group (37.6; CI = 32.2, 43.0).

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Comparison of Functional Outcomes Across Nonunion Assessment Methods

Blinded CT assessment identified more patients with a nonunion (18%) than did surgeon assessment (5%) (Table III). In the cohort for whom all data were available for all analyses (n = 370), 299 (62%) were found to have union with both assessment methods; 15 (4%), nonunion with both methods; 52, union with surgeon evaluation and nonunion with blinded CT assessment; and 4, union with blinded CT assessment and nonunion with surgeon evaluation. Patients who were found to have union with both assessment methods had the best functional outcomes, and patients deemed to have nonunion with both assessment methods had the poorest functional outcomes (Table IV).

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Demographics of the Study Cohort

The average age of the study cohort was approximately 57 years (Table V). When nonunion was diagnosed by blinded CT assessment (in 18% of the patients), BMI, smoking status, and work status differed significantly between the union and nonunion groups. The BMI was 2 kg/m2 higher in the nonunion group (mean and standard deviation, 32.6 ± 5.7 kg/m2) compared with the union group (30.4 ± 5.7 kg/m2); 67% of the nonunion group but only 45% of the union group had a BMI of >30 kg/m2 (p = 0.0011). Union was achieved in 68% of current smokers compared with 84% of non-smokers (p = 0.0198), 90% of employed patients compared with 75% of patients who were not working (p < 0.0001), 74% of patients with diabetes compared with 83% of patients without diabetes (difference not significant), and 84% of patients who underwent fusion of 1 joint, 79% of those who underwent fusion of 2 joints, and 78% of those who underwent fusion of 3 joints (difference not significant). For a patient to be included in the union group, all joints treated in that patient had to demonstrate union; hence, lower fusion rates in patients who underwent fusion of multiple joints were expected. In summary, nonunion was more likely in obese patients who smoked and were not working.

When nonunion was determined by surgeon assessment (5% nonunion rate), there were no significant differences in baseline characteristics between the union and nonunion groups.

When patient demographics were stratified according to the assessment method and union status (Table VI), BMI and work status differed significantly: i.e., nonunion was more likely in obese patients who were not working.

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Discussion

One year after ankle and/or hindfoot arthrodesis, 18% of the patients demonstrated nonunion on blinded CT assessment and 5.4% presented with nonunion based on surgeon evaluation that included clinical, radiographic, and CT parameters. Irrespective of the assessment method, patients with a nonunion had worse AOFAS-AHS and FFI (functional) scores than patients with successful fusion and lower SF-12 PCS scores, indicating an impact on quality of life. The differences between the union and nonunion groups for all 3 outcome measures were clinically meaningful24-26 regardless of the method used to assess nonunion. These findings illustrate the importance of achieving union following foot and ankle fusion procedures to attain a good outcome and a better quality of life. Patients with a nonunion according to blinded CT assessment were more likely to be overweight, smokers, and not working.

Review of these data suggests that future studies should use blinded CT assessment to define nonunion rather than rely on surgeon assessment. Surgeon assessment of nonunion is methodologically flawed as clinical outcome is a component. Furthermore, the concept of an asymptomatic nonunion is not supported by this study, as nonunion identified by CT imaging resulted in a worse outcome.

The functional outcomes were substantially better in the union group than in the nonunion group and are consistent with the literature. Studies of ankle arthrodesis demonstrated mean AOFAS scores of 67 points in 66 ankles after 9 years of follow-up28, 75.6 points in 21 patients after 35 months29, and 68.3 points in 16 patients with ≥6 months of follow-up30. A mean FFI of 17.6 points was reported for 34 patients at a median of 3.6 years after ankle arthrodesis31.

While nonunion is defined as incomplete osseous healing at ≥6 months following arthrodesis14, there is no consensus on the amount of osseous healing that constitutes “complete” versus “incomplete” healing or on how nonunions are assessed. A review of 12 studies published since 2000 that included a total of 961 ankle arthrodeses7,12,16-18,32-38 indicated that an average of 11% (range, 3% to 23%) of the procedures resulted in nonunion. Some authors defined nonunion on the basis of radiographic findings only16,18,35-37, whereas others relied on a combination of clinical and radiographic assessment7,12,17,32-34,38. Recently, CT has been proposed to provide a more sensitive, reliable, and accurate assessment of fusion13,15,39. Coughlin et al.13 suggested that 50% osseous bridging of the posterior facet of the subtalar joint on CT closely correlates with successful clinical outcomes. Dorsey et al.39 reported that clinically stable joints demonstrated fusion of ≥33.2% on CT, equivalent to the 50% osseous bridging recommended by Coughlin et al.13 if the same methodology is applied. Glazebrook et al.15 found a clinically important improvement in SF-12, FFI, and AOFAS-AHS scores in patients with at least 25% to 49% osseous bridging at fusion sites on CT, which is consistent with our results for patients assessed by blinded CT.

Despite the inclusion of patients with risk factors for nonunion (i.e., overweight, diabetes, smoking, substantial deformity, and intra-articular erosions) in the current study, the overall prevalence of nonunion was 5.4% at 1 year as diagnosed with surgeon assessment, compared with 11% for ankle and hindfoot arthrodeses reported in the literature7,12,16-18,32-38. The patients in our study were all treated with autologous bone graft or bone substitute to maximize fusion.

Nonunion following arthrodesis is likely determined by a combination of risk factors7,32. Nonunion following elective foot and ankle surgery is reportedly more frequent in smokers, in the presence of avascular bone at the arthrodesis site, and in patients in whom previous arthrodeses have failed7,40,41. Nonunion also appears to be more frequent in diabetic and obese patients4,42. In the current study, nonunion diagnosed with blinded CT assessment was found more often in patients who were overweight, smoked, or were not working.

A strength of this study is the use of prospectively collected clinical, radiographic, and CT data from a randomized, controlled, blinded, multicenter rhPDGF-BB trial, which ensured that all patients enrolled in the trial, rather than only those presenting with clinical symptoms, were evaluated for the presence of nonunion. Another strength is the large sample size of 389 ankle and hindfoot arthrodeses, the largest longitudinal study cohort to our knowledge.

This study also had limitations, including the relatively short follow-up period of 12 months. It is not clear whether the clinical results after nonunion may improve on longer follow-up or whether some cases of nonunion may deteriorate with time and cause more symptoms. The various arthrodesis sites in the patients in this study may have added confounding variables to the statistical analyses. In the analyses based on the surgeon’s composite evaluation of union, the small numbers in the nonunion group and subsequent low power may have precluded the determination of significant differences in baseline characteristics. Additionally, this assessment method introduces bias, as clinical outcome was one component of the evaluation.

In conclusion, this study demonstrated poorer 1-year functional results in patients with nonunion compared with patients who obtained fusion following ankle and hindfoot arthrodesis. Achieving solid osseous union is required for good functional outcomes.

NOTE: The following North American Orthopedic Foot and Ankle Study Group investigators participated in the original randomized controlled trial: Nicholas A. Abidi, MD; Jorge I. Acevedo, MD; John Anderson, MD; Robert Anderson, MD; Wayne Berberian, MD; Gregory C. Berlet, MD; Christopher Bibbo, DO; Donald Bohay, MD; Bradley J. Brainard, DO, MD; Bruce Cohen, MD; W. Hodges Davis, MD; Keith Donatto, MD; Hugh Dougall, MD, FRCSC; Mark E. Easley, MD; Andrew A. Elliott, MD; Adolph Samuel Flemister Jr., MD; William Granberry, MD; Justin Greisberg, MD; Steven L. Haddad, MD; Anthony Hinz, MD; Susan N. Ishikawa, MD; Juha I. Jaakkola, MD; Johnny Lau, MSc, MD, FRCSC; Ian Le, MD, FRCSC; Thomas Lee, MD; Sheldon S. Lin, MD; John Maskill, MD; G. Andrew Murphy, MD; Steven K. Neufeld, MD; M.J. O’Malley, MD; Murray Penner, MD, FRCSC; Terrence Philbin, DO; Steven Raikin, MD; Iain Russell, MD, FRCSC; Lew Schon, MD; James J. Sferra, MD; Naomi Shields, MD; Nebojsa Skrepnik, MD, PhD; Raymond Sullivan, MD; Michael Swords, DO; A. Brian Thomson, MD; Troy Watson, MD; and Kevin Wing, MD, FRCSC. The authors thank Dagmar Gross for assistance with preparation of the manuscript.

Investigation performed at the University of British Columbia in collaboration with the North American Orthopedic Foot and Ankle Study Group

Disclosure: One author was an employee of BioMimetic Therapeutics, Inc. (Wright Medical, Inc.) at the time that this study was conducted. The remaining authors received funding to cover research costs through an unrestricted grant and/or were working as consultants for BioMimetic Therapeutics, Inc. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work and “yes” to indicate that the author had a patent and/or copyright, planned, pending, or issued, broadly relevant to this work.

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