Abstract
Background:
Isolated subtalar arthrodesis is generally successful, with reported fusion rates of 84% to 100%. However, alteration of subtalar joint mechanics and talar body vasculature after ankle fusion may negatively influence subsequent ipsilateral subtalar joint fusion. Because there is very limited information on the subtalar fusion rate in patients with previous ipsilateral ankle fusion, the purpose of this study was to describe fusion rates in subtalar joint arthrodesis with and without preexisting ankle fusion in a large consecutive series of primary subtalar arthrodesis cases.
Methods:
All primary subtalar fusions performed between January 2000 and December 2010 were reviewed. Thirteen of 151 consecutive cases were in patients with existing ipsilateral ankle fusions. All patients were evaluated for clinical and radiographic evidence of nonunion at follow-up, and fusion rates in the groups with and without previous ipsilateral ankle fusion were compared.
Results:
Five nonunions occurred in the 13 cases with prior ipsilateral ankle arthrodesis, a 61.5% fusion rate. Twelve nonunions were identified in the 138 cases without prior ankle arthrodesis, a significantly higher fusion rate of 91.3% (P = .007).
Conclusion:
In our series, the subtalar fusion rate in patients with previous ipsilateral ankle arthrodesis was significantly lower than that for subtalar arthrodesis in the absence of ipsilateral ankle arthrodesis.
Level of Evidence:
Level III, retrospective comparative study.
Keywords
Subtalar arthrodesis is commonly recommended for the operative management of posttraumatic subtalar arthritis,1,4,7 complex acute calcaneal fractures, 3 primary subtalar arthritis, 15 symptomatic congenital deformities such as tarsal coalitions,9,15 posterior tibial tendon dysfunction, 12 and rheumatoid arthritis. 17 Reported fusion rates for open 10 and arthroscopic2,11 subtalar arthrodesis range from 84% to 100%.3,7,8,10-12,14,15
Several published studies note development or progression of subtalar osteoarthritis after ipsilateral ankle arthrodesis, in some cases necessitating fusion of the subtalar joint.5,13,16 Altered subtalar joint loading 18 and reduced talus vascularity 10 after ankle fusion may create a less favorable environment for subtalar fusion in this patient subgroup. However, the only study of which we are aware that quantified the subtalar fusion rate in patients with prior ipsilateral ankle fusion included just 6 such cases, too few for statistical analysis. 10 Identification of cases in which subtalar fusion may be particularly problematic is imperative so that the operative technique and postoperative care may be modified to maximize fusion rate. Accordingly, the goal of the retrospective study reported here was to determine the subtalar joint fusion rate in a larger group of primary subtalar fusion patients with preexisting ipsilateral ankle fusions, enabling valid statistical comparison to the fusion rate in patients from the same consecutive series who did not have ipsilateral ankle arthrodesis. We hypothesized that the fusion rate in patients with previous ipsilateral ankle fusion would be lower than that for subtalar arthrodesis in the absence of ipsilateral ankle arthrodesis.
Methods
In total, 151 consecutive primary subtalar arthrodesis cases in 149 patients performed between January 2000 and December 2010 were examined. Two experienced, fellowship-trained foot and ankle specialists performed all of the procedures. All patients had a symptomatic subtalar joint that did not respond to conservative treatment, which consisted of modification of activities or occupational status, nonsteroidal medications, use of orthotics or a brace, physical therapy, and subtalar joint steroid injection. Patient demographics are reported in Table 1.
Patient Characteristics. a
Values are presented as number (%) unless otherwise indicated.
Thirteen (8.6%) of these primary subtalar arthrodeses were in 13 patients (mean age, 56 years; range, 40-78 years) with existing successful ipsilateral ankle fusions. The operative indication for all 13 cases was degenerative subtalar arthritis subsequent to the ankle fusion (Figure 1). In these patients, the subtalar joint was fixed using 3 or more screws in 9 cases and 2 screws in 4 cases. One or more adjuvant treatments were employed in all 13 patients, consisting of autograft bone in 5, allograft bone in 8, and platelet-rich plasma in 11.
Patient with degenerative subtalar arthritis after a successful ankle arthrodesis.
The subtalar fusion rate in these patients was compared with the fusion rate in 136 patients (138 subtalar arthrodeses; 2 patients had both subtalar joints fused at different times) who underwent isolated primary subtalar arthrodesis without prior ipsilateral ankle arthrodesis (mean age, 49 years; range, 18-77 years). The operative indications for these patients were posttraumatic subtalar arthritis in 114, complex acute calcaneal fracture in 4, primary subtalar arthritis in 8, symptomatic congenital deformities such as tarsal coalitions in 9, and posterior tibial tendon dysfunction in 3. In this group, the subtalar arthrodesis was performed using 3 or more screws in 14 instances and 2 screws in 124 cases. Adjuvant treatment for arthrodesis was used in all patients in this group, with autograft bone used in 13 cases, allograft bone in 53 cases, demineralized bone graft in 3 cases, and platelet-rich plasma in 83 cases.
After surgery, all patients were discharged to home wearing a postoperative splint. Between the second and third weeks, after the operative wound had stabilized, a short-leg cast was applied and worn until 8 weeks after the surgery. At that time, a below-the-knee, prefabricated, removable fixed angle boot was applied. Patients were nonweightbearing for an average of 13 weeks (range, 12-20 weeks). During follow-up, patients were evaluated clinically and radiographically by an investigator not directly involved in the operative procedures. For both groups of patients, success of the subtalar fusion was determined using these criteria: (1) resolution of hindfoot pain with weightbearing and hindfoot stress and (2) bridging trabeculation at the subtalar joint based on lateral foot, mortise ankle, and Broden’s radiographs. In cases in which the clinical and radiographic evaluation was inconclusive, patients were further assessed with computerized tomography (CT) to categorize the fusion as successful or failed. 10 In these cases, the criterion for union was more than 50% posterior facet consolidation (Figure 2). 8
Computerized tomography scan showing nonunion of a subtalar arthrodesis after a successful ankle fusion.
Thirty-four cases required CT for unequivocal determination of fusion or lack of fusion. In cases in which subtalar fusion was readily achieved, patients received their final radiograph at 12 months and final follow-up appointment at 24 months. The average time between surgery and final radiograph was 30 months (range, 12-96 months) in the 13 cases with ipsilateral ankle fusion. Eleven patients in this group required CT to establish the presence or absence of fusion, with an average time between surgery and CT of 6 months. The average time between surgery and final radiograph was 20 months (range, 12-86 months) in the 138 cases without ipsilateral ankle fusion. Twenty-three required CT to determine the presence or absence of fusion. The average time between surgery and CT for these cases was 6 months.
The difference in fusion rates between the 2 groups was tested for statistical significance using the Fisher exact test with the significance level set at P = .05. A post hoc power analysis was performed to determine the statistical power of the fusion rate comparison between the patient groups with and without ipsilateral ankle fusion.
Results
Five nonunions were identified in the 13 cases with previous ipsilateral ankle arthrodesis, a subtalar fusion rate of 61.5%. Twelve of the subtalar arthrodeses in the 138 cases without previous ipsilateral ankle arthrodesis showed evidence of fusion failure, a 91.3% fusion rate (P = .007). Statistical power for this comparison was 70%. The mean follow-up for all patients was 34 months (range, 24-106 months), and the overall fusion rate was 88.7%.
In patients with an ipsilateral ankle fusion and a CT was performed, fusion was confirmed in 8 of those 11 cases, and lack of fusion was confirmed in the remaining 3. In the patients without ipsilateral ankle fusion where a CT was obtained, fusion was confirmed in 15 of the 23 patients, and lack of fusion was confirmed in the remaining 8.
Discussion
Subtalar arthrodesis has been reported to be a successful treatment for several hindfoot conditions, with high rates of fusion, high rates of satisfaction, and low complication rates.3,15 However, our clinical experience has led us to suspect that the subtalar joint fusion rate in patients with ipsilateral ankle fusion is lower, and therefore the significant 61.5% vs 91.3% dichotomy that was found was not surprising. Valderrabano et al 18 showed reductions in hindfoot inversion/eversion and internal/external rotation in cadaver feet with simulated ankle fusions and suggested that this phenomenon can increase the mechanical stress at the subtalar joint during ambulation. The increased mechanical stress may lead to diminished fusion rates, particularly if the subtalar joint is mechanically challenged as a result of weightbearing initiation prior to satisfactory bridging bony trabeculation.
In addition to the likelihood of increased mechanical forces on the subtalar joint following ipsilateral ankle fusion, it is believed that alterations in talar body vasculature after ankle fusion may inhibit fusion of the adjacent subtalar joint, 10 and the presence of avascular subchondral bone has been shown to be associated with subtalar joint fusion failure. To our knowledge, alterations in talar body vasculature following ankle arthrodesis have not been carefully studied; in theory, adding further preparation to the undersurface of the talar body after the dorsal surface has previously been prepared for fusion, particularly if the inferior talar neck vessels are violated, may diminish the chances for subtalar fusion.
The only other published study of which we are aware that documented subtalar fusion rates under this condition, by Easley and colleagues, 10 reported an overall union rate of 84% in 184 consecutive isolated subtalar arthrodesis cases using a lateral approach and 1 or 2 screws for rigid internal fixation. The patient cohort included only 6 cases in which the subtalar arthrodesis was adjacent to a prior ipsilateral ankle arthrodesis; of these 6 patients, 4 (66.7%) progressed to successful subtalar fusion. When these 6 cases were excluded, along with revision procedures and subtalar bone block distraction arthrodeses, the union rate improved to 90%. This corresponds closely to the 91.3% fusion rate documented in the present study. The authors stated that the patient group with ipsilateral ankle fusion was too small to allow statistical comparison of its subtalar fusion rate with that of the patients without adjacent ankle fusion but suggested that a dichotomy was possible. A Fisher exact test and retrospective power analysis performed on their reported primary arthrodesis fusion rates with and without ipsilateral ankle fusion found a P value of .200 and power of 21%. Our study, with 8 successful fusions and 5 nonunions among the patients with ipsilateral ankle fusion and statistical power of 70%, lends convincing support to the concept of reduced fusion likelihood in the presence of adjacent ankle fusion.
It was initially intended that our analysis of the effect of ipsilateral ankle fusion would include the influence of additional patient factors in an effort to identify those predictive of subtalar joint fusion success or failure, but the group sizes available for evaluation of the respective factors were inadequate.
Bone graft is commonly used in subtalar arthrodesis,1,3,4,7 and we used allograft or autograft bone graft as an adjuvant in the majority of cases reported here. Easley et al, 10 in their aforementioned study, showed that subtalar fusion rate was not related to the use of bone graft in their overall patient cohort, nor was the type of bone graft (allograft or autograft) a determining factor. They did detect a negative impact of smoking on subtalar joint fusion rate in their overall patient cohort, with a 92% to 73% fusion rate ratio for smokers and nonsmokers, respectively. The influence of these factors on subtalar fusion in patients with ipsilateral ankle fusion remains to be determined, along with that of age, sex, diabetes, rheumatoid arthritis, fixation method, and adjuvant platelet-rich plasma use.
In addition to the relatively small number of patients with ipsilateral ankle fusion, we acknowledge further limitations of this investigation: it was a retrospective analysis; the number of screws used for subtalar joint fixation was not standardized; bone grafting, when employed, was not standardized; and CT was limited to only those patients with suspected nonunions. While we recognize that CT is optimal for confirmation of hindfoot fusion, 6 in our opinion, the accompanying radiation exposure and expense are unwarranted in patients with functional improvement and pain relief.
In summary, the present study was undertaken because the largest previously reported series of subtalar arthrodeses that included cases in which ipsilateral ankle fusion was present contained an insufficient number of cases to allow a statistically supported conclusion regarding a possible diminished union rate. Our series included a larger number of these cases, allowing statistical analysis that clearly demonstrated a markedly lower subtalar fusion rate in patients with previous ipsilateral ankle fusion. As a result of this finding, we routinely advise our patients with ipsilateral fused ankles that reported favorable subtalar fusion rates may not be achieved in their surgery. Moreover, we educate other physicians about our experience and the rationale for not treating subtalar arthrodesis in patients with prior ipsilateral ankle arthrodesis as routine fusions.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.