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. Author manuscript; available in PMC: 2023 Mar 2.
Published in final edited form as: Foot Ankle Int. 2022 Mar 16;43(6):818–829. doi: 10.1177/10711007221078001

Total Ankle Replacement Provides Symmetrical Postoperative Kinematics: A Biplane Fluoroscopy Imaging Study

Amy L Lenz 1,2,3, Rich J Lisonbee 1,2, Andrew C Peterson 1,2,3, Koren E Roach 1,2,4, K Bo Foreman 5, Alexej Barg 1,6,7, Andrew E Anderson 1,2,5,8
PMCID: PMC9980879  NIHMSID: NIHMS1772740  PMID: 35293257

Abstract

Background:

In vivo measurements of tibiotalar and subtalar joint motion following TAR are unavailable. Using biplane fluoroscopy, we tested the hypothesis that the prosthetic tibiotalar joint and adjacent subtalar joint would demonstrate kinematic and range of motion differences compared to the contralateral untreated limb, and control participants.

Methods:

Six patients of 41 identified candidates that all underwent unilateral Zimmer TAR (5.4 ± 1.9 years prior) and 6 control participants were imaged with biplane fluoroscopy during overground walking and a double heel-rise activity. Computed tomography scans were acquired; images were segmented and processed to serve as input for model-based tracking of the biplane fluoroscopy data. Measurements included tibiotalar and subtalar kinematics for the TAR, untreated contralateral, and control limbs. Statistical parametric mapping quantified differences in kinematics throughout overground walking and the double heel-rise activity.

Results:

Patients with this TAR performed walking and heel-rise activities symmetrically with no significant kinematic differences at the tibiotalar and subtalar joints between limbs. Compared to control participants, patients exhibited reduced dorsi/plantarflexion range of motion that corresponded to decreased peak dorsiflexion, but only in the late stance phase of walking. This reduction in tibiotalar dorsi/plantarflexion range of motion in the TAR group became more apparent with double heel-rise activity.

Conclusion:

Patients with a Zimmer TAR had symmetric kinematics during activities of walking and double heel-rise, but they did exhibit minor compensations in tibiotalar kinematics as compared to controls.

Clinical Relevance:

The lack of significant kinematic compensation at the subtalar joint may explain why secondary subtalar osteoarthritis is reported as being relatively uncommon in patients with some TAR designs.

Keywords: total ankle replacement, subtalar joint, biplane fluoroscopy, kinematics

Introduction

Tibiotalar osteoarthritis (OA) affects 100,000 individuals per year.11 Most cases of tibiotalar OA are secondary to trauma,40 and patients are typically younger than those with knee OA.32 Tibiotalar fusion1 and total ankle replacement (TAR)3,4,10,13,18,37,38,41,50 are currently the only effective treatments for end-stage tibiotalar OA. Ankle replacement is an attractive alternative to fusion because it allows for tibiotalar motion. Still, despite improvements in TAR designs, survival rates are substantially lower than knee or hip arthroplasty22; the overall survivorship is 89% at 10 years with an annual failure rate of 1.2%.50 TAR failure may in part be caused by altered biomechanics at the prosthetic joint as well as adjacent subtalar joint. Yet, in vivo measurements of tibiotalar and subtalar joint motion following total ankle replacement are not available.

A comparative analysis of tibiotalar and subtalar joint motion between patients with a TAR, their contralateral untreated limb, and control participants would help to identify potential biomechanical compensations that could contribute to unsatisfactory clinical outcomes. To date, only skin-based optical motion capture studies have evaluated motion of patients with TAR. Results have demonstrated reduced range of motion (ROM) of the surgical TAR limb.9,16,17,33,42 However, these studies evaluated motion of the tibia relative to the calcaneus (ie, tibiocalcaneal) because one cannot differentiate tibiotalar and subtalar movement separately using optical motion capture with no suitable location to place a marker on the talus.49

Biplane fluoroscopy accurately quantifies kinematics through the registration of volumetric computed tomography (CT) data with images acquired in vivo by 2 fluoroscopes,5 which enables direct measurement of individual bone motion.23 Herein, we used biplane fluoroscopy to measure in vivo tibiotalar and subtalar motion in patients who had undergone a unilateral TAR. We hypothesized that (1) the prosthetic tibiotalar joint would demonstrate differences in tibiotalar kinematics and decreased ROM as compared with the contralateral untreated tibiotalar joint, and the tibiotalar joint in control participants, and (2) the subtalar joint on the limb with a TAR would also demonstrate differences in subtalar kinematics or ROM as compared to the contralateral untreated subtalar joint, and the subtalar joint in control participants.

Materials and Methods

Participant Recruitment and Screening

Approval from the institutional review board (no. 65620) was granted to recruit patients with a unilateral TAR and healthy individuals to serve as a control group. All participants completed verbal and written informed consent.

First, a database search identified 41 consecutive patients who had undergone a Zimmer Trabecular Metal Total Ankle Replacement at our surgical center over a 4-year period. The Zimmer TAR was selected because of a low rate of aseptic loosening of prosthetic components and functional outcomes noted by superior postoperative range of motion.3,21 All surgeries were performed by a single fellowship-trained orthopaedic foot and ankle surgeon. Medical charts were reviewed to exclude patients who had died, were >95 years old, and/or had comorbidities, including chronic pain, peripheral neuropathy, rheumatoid arthritis, stroke, muscular dystrophy, Charcot-Marie-Tooth disease, or a body mass index (BMI) of >40. Twenty-four individuals were identified and 10 showed initial interest; 4 individuals were unable to travel for testing, leaving 6 participants. These 6 patients underwent radiographic evaluation with weightbearing radiographs of the foot and ankle (anteroposterior, hindfoot, lateral, and mortise) to evaluate the limb treated with TAR for appropriate alignment, implant loosening, and subtalar OA using the Kellgren-Lawrence (KL) scale.20 Using radiographic films, subtalar OA in the contralateral, nonsurgical limb was also evaluated with the KL scale. Radiographic evaluation was performed by a musculoskeletal radiologist not involved in the surgeries.

Healthy adults were recruited to serve as a control group. The KL scale evaluated any degenerative changes in the tibiotalar and subtalar joints. Participants were excluded if either joint of interest had a KL score >1. None were excluded based on these criteria.

To our knowledge, separate kinematics of the tibiotalar and subtalar joints reported as joint angles in the 3 planes of motion (sagittal, coronal, and transverse) have not been quantified following TAR. Sample data of this kind are needed for an a priori sample size calculation and, therefore, a formal power analysis could not be conducted prior to recruitment. Thus, our sample was one of convenience (n = 6 patients, n = 6 healthy individuals), which provided a Cohen d of 1.9 that is representative of a large effect size.

Biplane Fluoroscopy Motion Capture

Biplane fluoroscopy was used to measure tibiotalar and subtalar kinematics.6,19,48 The system consisted of 2 separately mounted pairs of x-ray emitters and image intensifiers (Radiological Imaging Services) positioned approximately 90 degrees to one another with a 110-cm focal length distance to image overground walking. The system surrounded 2 force plates (AMTI OR6 series) that identified gait events (eg, heel-strike, toe-off); these force recordings were temporally synced with the biplane fluoroscopy system using an external trigger.23

Two activities were performed by all participants: (1) overground barefoot walking was selected as a representative activity of daily living, and (2) a double heel-rise activity aimed to evaluate the full-weighted ROM. High-speed video from the biplane fluoroscopy system was collected at 200 Hz (Figure 1A). To account for limb size, bony morphology, and soft tissue mass across participants, we applied a range of energy settings for the biplane fluoroscopy system of 64 to 80 kVp and 1.2 to 2.4 mAs. Two trials of each activity at a self-selected speed were recorded. Total fluoroscopy exposure time for each participant did not exceed 3 minutes, which is equivalent to an effective dose of 5.2 × 10−2 mSv (~9.4 days of background radiation). One trial for each activity was processed to calculate kinematics.

Figure 1.

Figure 1.

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(A) Biplane fluoroscopy data were collected in 2 calibrated views during dynamic activities (walking shown). (B) Medical imaging consisted of a foot and ankle CT with metal artifact reduction algorithms applied for patients with a TAR. (C) Segmentation was performed to reconstruct 3-dimensional bones adjacent to the TAR implants, avoiding regions with metal artifact present. (D) CAD models were imported, and DRRs (shown in green) were created for the bone and CAD models separately. The DRRs were statically tracked in a standing trial to establish an alignment between the bone and the TAR implants. (E) The bone and CAD models were transformed into the CT coordinate system to resegment the bone-implant interface to create a solid hybrid model of both components. (F) The hybrid model was then used to track dynamic trials (overground walking and heel-rise activities). (G) Tibiotalar and subtalar kinematics were normalized from heel-strike to toe-off for walking. CAD, computer-aided design; CT, computed tomography; DRRs, digitally reconstructed radiographs; TAR, total ankle replacement.

CT, Model-Based Markerless Tracking

Bilateral CT images were acquired for each participant (SOMATOM Definition AS; Siemens Medical Solutions) from the distal foot to the proximal tibia with a 512 × 512 acquisition matrix field of view, and a 0.6-mm slice thickness with isotropic voxel size. Tube voltage and current averaged 90 kVp and 45 mAs, respectively, using CareDose. An iterative metal artifact reduction algorithm was applied to scans with an implant (Siemens iMAR) (Figure 1B). The CT scan effective dose equivalent of ionizing radiation for each participant was 0.9 mSv, which is approximately 29% of the background radiation an average person in the United States receives annually from naturally occurring sources (approximately 3.1 mSv per year).

The CT images were segmented semiautomatically (Amira v6.0; Visage Imaging) to generate 3-dimensional surfaces representing the tibia, talus, and calcaneus. For patients with a TAR, initial segmentation included the bone boundary adjacent to the TAR implants (Figure 1C). A published methodology was then followed to minimize the effect CT metal artifact had on the accuracy of the segmentation. Briefly, this method uses CAD implant models (provided by the manufacturer) to create a distinct boundary for tracking6 (Figure 1D) and includes a resegmentation step to eliminate the effect of artifact at the bone-implant interface (Figure 1E) prior to dynamic tracking for kinematics (Figure 1F).48

Dynamic Tibiotalar and Subtalar Kinematics With Range of Motion

Coordinate systems were established using a geometric and landmark-based approach for the tibia, talus, and calcaneus.6,48 A weightbearing neutral position of the ankle was determined from a static standing biplane fluoroscopy trial; the talus and calcaneus were aligned with respect to the tibial coordinate system. During all completed activities, dynamic joint angles (kinematic motion of one bone relative to the other in 3 rotational planes) were calculated for the tibiotalar and subtalar joints as previously validated.48 Tibiotalar and subtalar joint angles were reported for dorsi/plantarflexion, inversion/eversion, and internal/external rotation motions. Ground reaction forces were used to identify heel-strike and toe-off to normalize kinematics as a percentage of stance (Figure 1G). Double heel-rise activities were normalized using the dorsi/plantarflexion tibiotalar joint angles to identify 2 inflection points at the beginning and end of activity completion defined by the second derivative (where accelerations equaled zero) and reported as percentage activity. Tibiotalar and subtalar ROM was calculated for all activities as the within-trial kinematic maximum minus the minimum.

Statistical Analysis

All statistical tests were performed in MATLAB (MathWorks). The Kolmogorov-Smirnov test25 was performed and yielded a normally distributed data set; therefore, parametric tests were employed. The mean and 95% CIs were plotted to visualize group profiles (TAR, contralateral untreated, and controls) with respect to normalized percentage stance for tibiotalar and subtalar kinematics. Differences in tibiotalar and subtalar ROM were evaluated within limbs (TAR vs untreated) using a paired Student t test, and a Student t test for between groups (TAR vs controls, and untreated vs controls). Statistical parametric mapping (SPM) was applied to compare kinematics at each instant of normalized percentage stance or percent activity (1d version 0.4; MATLAB-based open source software, www.spm1d.org)27–31 between the TAR, untreated side, and control limbs.

Results

In total, 6 patients with unilateral TAR and 6 healthy individuals were included (Table 1). The mean age was 68.2 years for patients with a TAR (range, 54–79 years), and the mean BMI was 28.9 (range, 21.3–39.5). Two patients had a TAR on the right side, 4 had one on the left. The mean time from surgery to testing (and SD) was 5.4 ± 1.9 years, with all participants having had the procedure at least 18 months (range, 1.6–6.6 years) previously. Among patients, KL scores for the subtalar joint on the surgical limb ranged from 1 (doubtful) to 4 (severe) and 0 (none) to 2 (minimal) on the contralateral, nonsurgical limb. For healthy participants, the mean age was 27.0 years (range, 22–33 years), and the mean BMI was 23.0 (range, 19.8–27.7). Control participants were significantly younger than patients with TAR (P < .001), but BMI was not significantly different (P = .07).

Table 1.

Participant Demographics.

Case Treated Side Sex, Age (y) Height (cm) Body Mass (kg) BMI Years Postoperation Primary Etiology
TAR 1 Left M, 77 177.8 88.9 23.1 1.6 Posttraumatic
TAR 2 Right F, 72 170.2 88.5 30.5 6.6 Posttraumatic
TAR 3 Left M, 79 177.8 101.6 32.1 6.4 Posttraumatic
TAR 4 Left M, 63 172.7 117.9 39.5 6.5 Posttraumatic
TAR 5 Left F, 64 172.7 63.5 21.3 6.0 Posttraumatic
TAR 6 Right M, 54 182.9 90.7 27.1 5.5 Posttraumatic
Control 1 — F, 28 172.0 63.5 21.5 — —
Control 2 — M, 33 186.0 87.5 25.3 — —
Control 3 — F, 27 169.0 56.5 19.8 — —
Control 4 — M, 22 183.0 75.0 22.4 — —
Control 5 — M, 28 187.0 97.0 27.7 — —
Control 6 — F, 24 161.0 54.4 21.0 — —
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Abbreviations: BMI, body mass index; TAR, total ankle replacement.

Dynamic Tibiotalar and Subtalar Kinematics

During walking, patients with TAR had symmetric kinematics between the treated and untreated limbs for both the tibiotalar (Figure 2) and subtalar joints (Figure 3). When comparing the TAR side to control participants, the tibiotalar joint in patients with a TAR demonstrated significantly reduced dorsiflexion from 63.9% to 69.7% of late stance (Figure 2), and the subtalar joint demonstrated significantly greater dorsiflexion and eversion from 79.5% to 85.7% of late stance (Figure 3). The untreated side of patients demonstrated minimal compensations compared to control participants, with no significant differences in the tibiotalar joint and only a small region of significantly greater eversion in the subtalar joint from 84.4% to 85.7% of late stance (Figure 3). Although internal and external rotation kinematics were not significant, greater rotational variability was seen in the TAR limb. There was no significant difference in walking speed between patient and control groups (P = .86). Notably, patients with a TAR performed overground walking at an average (SD) self-selected speed of 1.6 ± 0.2 m/s (range, 1.5–2.0 m/s), and participants in the healthy control group performed at an average (and SD) speed of 1.7 ± 0.3 m/s (range, 1.3–2.0 m/s).

Figure 2.

Figure 2.

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Line graph showing tibiotalar kinematics (dorsi/plantarflexion, inversion/eversion, and internal/external rotation) during walking for the limbs treated with TAR (blue), the contralateral, untreated limbs (red), and the control participants (green). Results are normalized as the percentage of stance (with 0% indicating initial heel contact and 100% indicating toe off). Portions of the stance phase of walking during which differences were significant (*) as evaluated with statistical parametric mapping are shown with a horizontal bar. The shaded regions indicate the 95% CIs.

Figure 3.

Figure 3.

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Line graph showing subtalar kinematics (dorsi/plantarflexion, inversion/eversion, and internal/external rotation) during walking for the limbs treated with TAR (blue), the contralateral, untreated limbs (red), and the control participants (green). Results are normalized as the percentage of stance (with 0% indicating initial heel contact and 100% indicating toe-off). Portions of the stance phase of walking during which differences were significant (*) as evaluated with statistical parametric mapping are shown with a horizontal bar. The shaded regions indicate the 95% CIs.

During the double heel-rise activity, patients with TAR had symmetric kinematics between the treated and untreated limbs for both the tibiotalar (Figure 4) and subtalar joints (Figure 5). The TARs demonstrated significantly reduced dorsiflexion from 0 to 6.4% and 90.6% to 100%, reduced plantar flexion from 43.3% to 62.5%, and increased external rotation from 39.1% to 50.1% and 62.9% to 64.7% of the activity when compared to control participants (Figure 4), but no significant differences in subtalar kinematics were observed (Figure 5). The untreated side of patients demonstrated compensations when compared to controls at the beginning and end of the heel-rise activity, with significantly reduced dorsiflexion from 0% to 2.6% and 92.6% to 100% (Figure 4), but no significant differences in subtalar kinematics were observed (Figure 5).

Figure 4.

Figure 4.

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Line graph showing tibiotalar kinematics (dorsi/plantarflexion, inversion/eversion, and internal/external rotation) during double heel-rise for the limbs treated with TAR (blue), the contralateral, untreated limbs (red), and the control participants (green). Results are normalized as the percent of stance (with 0% indicating initial heel contact and 100% indicating toe off). Portions of the stance phase of walking during which differences were significant (*) as evaluated with statistical parametric mapping are shown with a horizontal bar. The shaded regions indicate the 95% CIs. TAR, total ankle replacement.

Figure 5.

Figure 5.

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Line graph showing subtalar kinematics (dorsi/plantarflexion, inversion/eversion, and internal/external rotation) during double heel-rise for the limbs treated with TAR (blue), the contralateral, untreated limbs (red), and the control participants (green). Results are normalized as the percentage of stance (with 0% indicating initial heel contact and 100% indicating toe-off). The shaded regions indicate the 95% CIs. TAR, total ankle replacement.

Range of Motion

During walking, tibiotalar and subtalar ROM demonstrated 3 significant comparisons including decreased dorsi/plantarflexion and internal/external rotation (Table 2). During the double heel-rise activity, the tibiotalar ROM demonstrated 7 significant comparisons (Table 2). Subtalar ROM demonstrated no significant differences during the heel-rise activity (Table 2).

Table 2.

Tibiotalar and Subtalar Joint Angle Range of Motions (ROMs) for Walking and Double Heel-Rise.a

A) Tibiotalar Range of Motion, degrees
Walking Double Heel-rise
TAR Untreated Controls TAR Untreated Controls
Dorsi/plantarflexion 10.3 ± 3.1 13.6 ± 4.5 17.8 ± 4.6 14.8 ± 5.7 24.7 ± 6.3 34.4 ± 9.5
Eversion/inversion 2.5 ± 0.6 2.9 ± 1.0 3.8 ± 1.7 2.3 ± 1.3 3.0 ± 1.4 6.6 ± 2.9
External/internal rotation 4.8 ± 1.7 8.9 ± 3.2 5.3 ± 1.3 5.6 ± 0.8 6.7 ± 2.0 10.9 ± 3.3
B) P Values
Walking Double Heel-rise
TAR vs Untreated TAR vs Controls Untreated vs Controls TAR vs Untreated TAR vs Controls Untreated vs Controls
Dorsi/plantarflexion 0.31 0.01 0.14 0.04 0.0001 0.03
Eversion/inversion 0.54 0.12 0.28 0.45 0.001 0.005
External/internal rotation 0.02 0.55 0.04 0.30 0.0005 0.006
C) Subtalar Range of Motion, degrees
Walking Double Heel-rise
TAR Untreated Controls TAR Untreated Controls
Dorsi/plantarflexion 3.3 ± 1.6 5.8 ± 2.0 4.6 ± 1.5 5.6 ± 2.2 5.4 ± 2.1 5.5 ± 3.1
Eversion/inversion 4.1 ± 2.0 8.5 ± 1.6 7.6 ± 2.4 4.2 ± 2.0 5.1 ± 2.9 6.7 ± 3.0
External/internal rotation 4.7 ± 1.5 5.9 ± 1.8 6.5 ± 2.1 6.7 ± 3.8 6.2 ± 2.6 7.4 ± 2.4
D) P Values
Walking Double Heel-rise
TAR vs Untreated TAR vs Controls Untreated vs Controls TAR vs Untreated TAR vs Controls Untreated vs Controls
Dorsi/plantarflexion .003 .18 .29 .86 .90 .98
Eversion/inversion .009 .02 .44 .46 .07 .30
External/internal rotation .06 .13 .61 .72 .73 .41
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Abbreviation: TAR, total ankle replacement limb.

a

Untreated: untreated contralateral limb; Controls: healthy limbs from a population with no history of ankle injuries or surgery. The values are given as the mean and SD. Bold text indicates significant findings for the t-test comparisons between noted limbs.

Discussion

Modern TAR implants typically include bicondylar geometry to mimic the shape of the native tibiotalar joint, but until now, kinematic function following ankle arthroplasty was largely unknown.39 Herein, we found the TAR provided 6–degree of freedom motion at the prosthetic tibiotalar joint, which is consistent with the native tibiotalar joint as noted in the findings of Roach et al34 that demonstrated considerable inversion/eversion and internal/external rotation motion (in addition to expected dorsi/plantarflexion) in control participants. Additionally, patients with TAR had symmetric kinematics with only minor losses of ROM at the subtalar joint, and only minor deviations compared to control participants. Overall, kinematics of the subtalar joint in patients who underwent a TAR were closer to normal kinematics than patients with a tibiotalar arthrodesis that demonstrated increased plantarflexion and decreased eversion during early stance of walking.23 This may explain why subtalar joint OA progression is typically not observed in the majority of patients following TAR, with lower rates of adjacent joint complications.14,35 Collectively, we view these biomechanical findings as being a favorable outcome of TAR in this patient group. We did, however, observe increased variability in the implanted tibiotalar internal/external rotation kinematics in the TAR limb, which we posit is due to the imprecision of positioning the talar implant in the axial plane for this specific Zimmer TAR implant. Notably, coronal and sagittal plane alignment of the talar implant is evaluated during surgery using fluoroscopy with the tibia fixed in a frame, but alignment of the talar component in the axial plane is not visualized. Additionally, the lateral surgical approach used in the Zimmer TAR is different than all other TAR implants, also making this observation specific to this implant. Accordingly, additional research that includes pre- and postoperative comparisons with longitudinal monitoring of kinematics is needed to comprehensively understand the effects of surgery.

By our orthopaedic surgeons, positive surgical outcomes following TAR are clinically indicated by (1) a reduction of pain level by at least half,26 (2) return to normal activities and possibly recreational activities,8,45,46 (3) preservation of preoperative ROM (but not necessarily a return to normal ROM),36 and (4) mid- to long-term slowing of secondary subtalar joint degeneration.43 By this clinical impression list, combined with our kinematic and ROM results, our patient cohort would be determined to have demonstrated a positive outcome at this single postoperative evaluation. No patients reported pain at the time of study, and all were able to perform normal activities, with some patients (n = 3) still performing recreational activities (eg, biking, skiing, hiking). We interpret the minor reductions in ROM at the prosthetic tibiotalar joint to be of minimal clinical concern because patients are typically not expected to regain full (ie, preinjury) ROM after TAR, yet an improvement in tibiotalar ROM compared to preoperative function is desirable.17

Direct comparisons of our data are not possible because no biplane fluoroscopy studies have evaluated TAR, nor have the biomechanics of this Zimmer TAR design been evaluated in vivo. Still, our findings do agree with Brodsky et al, who performed gait analysis studies on patients with a unilateral Scandinavian Total Ankle Replacement (STAR) and reported sagittal plane ROM of 12.7 ± 4.2 degrees for the arthroplasty side compared to 17.3 ± 3.5 degrees on the untreated limb.9 These values are slightly higher than our tibiotalar ROM results because skin-based motion capture considers only the combined contributions of the tibiotalar and subtalar joints.12,44 Another study using the Milwaukee Foot Model in patients with a Salto Talaris implant demonstrated reduced hindfoot ROM in terminal stance to the pre-swing phase of gait on the TAR side, which is consistent with our findings.17

In older adults, the metabolic energy cost of walking (MECW) curve relative to optimal speed is shifted upward, indicating increased age is associated with less economical gait.24 From a functional perspective, older adults with a decreased MECW are associated with improved gait speed and mobility.47 In our study, no statistical differences were noted between the 2 cohort’s walking speeds. In fact, patients with TAR herein walked faster (average: 1.6 m/s) than the healthy adult walking speed for similarly aged participants (1.10–1.55 m/s).2,7 Asymmetric gait has been shown to require up to 80% more metabolic power,15 but because our patients walked symmetrically we posit that there was minimal increased metabolic cost as a result of TAR surgery.

Our study has limitations that warrant discussion. No preoperative analysis was conducted in these patients to evaluate joint stiffness, health, or clinical ROM measures to determine if preoperative ROM was maintained. Owed to the time- and labor-intensive process of analyzing biplane fluoroscopy images, and to reduce radiation exposure, only 2 activities were investigated. Future studies could consider additional activities of daily living or recreational activities. Still, inclusion of the double heel-rise activity was viewed as a strength of this research because it demanded a larger ROM requirement than walking. Additionally, there is a need to study relationships between kinematics and patients’ etiology of arthritis, along with investigating different designs of TAR in a potential future pre- and postoperative longitudinal study design. A longitudinal study would also be able to further investigate the long-term risk of developing subtalar OA in patients with a TAR. Furthermore, of the 41 patients screened and 10 that showed interest, there may be a selection bias for those who were performing well postoperatively as no one reported pain or negative function at the time of recruitment. Also, our control participants were significantly younger. Future studies should recruit age-matched controls. Lastly, we recognize a larger sample size will be needed to generalize findings and make definitive conclusions regarding the biomechanics of TAR.

Conclusions

In conclusion, when compared to control participants, patients with a Zimmer TAR demonstrated minimal reductions in ROM and minor differences in tibiotalar kinematics during the late stance of walking; these differences were accentuated when the double heel-rise activity was analyzed. Collectively, this suggests that this Zimmer TAR provides near-normal postoperative motion at the prosthetic joint as assessed during walking and a heel-rise activity, but other TAR implant designs may perform differently. The observation of symmetric tibiotalar and subtalar kinematics between treated and untreated limbs could also be viewed as a positive clinical outcome because symmetric gait reduces energy expenditure. Nevertheless, additional research that includes pre- and postoperative comparisons with longitudinal monitoring of kinematics is needed to comprehensively understand the effects of this surgery.

Acknowledgments

Beth O’Donnell is acknowledged for assistance with CT scans and application of iMAR. Dr. Albert Burstein and Dr. Charles Saltzman are thanked for their research consultation.

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Andrew E Anderson, PhD, reports receiving all support for the present manuscript (eg, funding, provision of study materials, medical writing, article processing charges, etc; no time limit for this item) from the National Institutes of Health, including grants or contracts.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The institution of one or more of the authors has received funding from the National Institutes of Health (R21 AR069773), the Stryker/ORS Women’s Research Fellowship, and the L.S. Peery Discovery Program in Musculoskeletal Restoration.

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