*3.1. Demographics*

Demographic characteristics of the patient group and control group are presented in Table 1. The two groups had a similar age, length, and gender ratio. The walking speed did not differ significantly between the two groups. The TAF group had a significantly higher body mass index (BMI) compared to the control group (*p* < 0.001). The BMI was therefore considered as a confounding factor when analyzing the kinematic differences between these two groups. Gait analysis was performed for 29 months after surgery on average. Five patients were treated with a syndesmotic screw, because insufficient syndesmotic stability of the distal tibiofibular joint was achieved with the plate osteosynthesis of the posterior malleolus alone. The mean clinical follow-up lasted 101 weeks (range: 60–171). Seven of the fifteen patients underwent some kind of implant removal. One patient had a superficial wound infection after implant removal. One patient developed a chronic regional pain syndrome (CRPS). No non-unions or mal-unions were reported. Two patients showed progression towards tibiotalar osteoarthritis on the latest available X-rays.

**Table 1.** Baseline characteristics of participants.


Continuous variables are expressed as means and ranges and categorical variables are presented as numbers and percentages. Abbreviations: L, left; R, right.

#### *3.2. Patient-Reported Outcome Scores*

The AOFAS, EQ-5D, and EQ-5D VAS scores are displayed in Table 2 with missing data for three patients [7]. The patients scored an average of 78 on the AOFAS survey, reflecting a good outcome after surgery (e.g., 100 = excellent score). For the first domain of the EQ-5D, mobility, the majority (67%) of the patient group (TAF) had moderate problems. None of the 12 patients reported self-care problems. The minority of patients (42%) suffered problems with daily activities, whereas two-thirds of the patients experienced moderate pain (67%). The presence of depression/anxiety was found to be negligible.


**Table 2.** Patient-reported outcomes after trimalleolar ankle fracture osteosynthesis.

Categorical variables are expressed as numbers and percentages. The AOFAS is expressed as the mean and range. All questionnaires were standardized and tested for reproducibility and validated for the Dutch language. Three of the fifteen patients did not report their outcome scores. Abbreviations: AOFAS, American Orthopedic Foot and Ankle Society; EQ-5D, EuroQol 5 dimensions; VAS, visual analog scale.

#### *3.3. ROM Comparisons of the Affected and Contralateral Side in the Patient Group*

During the loading response, the ankle segment of the affected side presented a significantly reduced ROM in the sagittal plane (*p* < 0.0125, large effect) and a trend towards reduced frontal plane ROM (*p* = 0.049, medium effect) (Table 3, Figure 1). Furthermore, the Chopart joint of the affected foot showed a trend towards reduced frontal plane ROM during the loading response (*p* = 0.013) and a trend towards an increased ROM during the midstance phase (*p* = 0.03). No significant differences were observed during the terminal stance phase. During the pre-swing phase, the mean ankle segment sagittal and transverse plane ROM was significantly reduced (*p* < 0.0125) in the affected foot, with mean differences of −3.4◦ and −1.4◦ and effect sizes of d = −0.9 (large) and d = −0.5 (medium), respectively.


**Table 3.** Patient group ROM of the affected foot and non-affected contralateral sides.

*p* values represent the outcome of the paired *t*-test (α = 0.05); significance: *p* < 0.0125; when data were not normally distributed, the *p* values represent the outcome of the Wilcoxon test. Effect size represents the Cohen's d calculated for the *t*-test and r-value for the Wilcoxon test. Abbreviations: MTP 1, first metatarsophalangeal joint; DF/PF, dorsal flexion–plantar flexion (sagittal plane); Inv/Eve, inversion–eversion (frontal plane); Add/Abd, adduction–abduction (transverse plane).

## *3.4. Comparison ROM Affected Side in the Patient Group versus Control Group*

During the loading response, the transverse plane ankle segment ROM was significantly (*p* < 0.0125) lower in the affected side (1.3 ± 0.6) compared to the control group (3.1 ± 1.1), with a large effect (η<sup>2</sup> = 0.37) (Table 4, Figure 1). During the midstance phase, the affected foot showed a reduced transverse plane ROM at the Chopart joint; hence, this did not reach a significance level. During the terminal stance phase, the affected

foot demonstrated a reduced (*p* = 0.045) sagittal plane ROM at the ankle segment and an increased ROM in the Chopart joint (*p* = 0.038).

**Figure 1.** Kinematic waveforms of the ankle, Chopart, Lisfranc, and MTP 1 joints. Abbreviations: DF, dorsal flexion; PF, plantar flexion; Inv, inversion; Ev, eversion; Add, adduction; Abd, abduction.

During the pre-swing phase, the affected foot demonstrated a significantly reduced transverse plane ROM compared to the control group (*p* = 0.011, medium effect). Moreover, a clear trend towards reduced ROM was observed in other joints (ankle segment and Chopart joint) and planes (sagittal and transverse) as well.


**Table 4.** ROM of the affected foot versus the control group.

*p* values represent the outcomes of the one-way ANOVA test. BMI had no significant effect except a trend towards significance for Lisfranc Inv/Eve during pre-swing (*p* = 0.035); significance: *p* < 0.0125, trend to significance. Abbreviations: MTP 1, first metatarsophalangeal joint; DF/PF, dorsal flexion–plantar flexion (sagittal plane); Inv/Eve, inversion–eversion (frontal plane); Add/Abd, adduction–abduction (transverse plane).

#### *3.5. Comparison Joint Coupling for the Three Cohorts*

The cross-correlation coefficients showed a small joint coupling for the patient group in both feet when compared to the control group (Table 5). The largest differences were seen for the following inter-segment rotations: ankle inversion–eversion with ankle adduction–abduction, ankle inversion–eversion with forefoot dorsiflexion–plantarflexion, and ankle inversion–eversion with forefoot inversion–eversion. For these inter-segment rotations, the patient group showed a medium joint coupling ((−) 0.3 to (−) 0.69) for

the non-affected and affected side, while the control group showed large joint coupling (>0.7 or <−0.7).

**Table 5.** Cross-correlation for all three cohorts.


Abbreviations: DF/PF, dorsal flexion–plantar flexion (sagittal plane); Inv/Eve, inversion–eversion (frontal plane); Add/Abd, adduction–abduction (transverse plane).

## **4. Discussion**

In this study, the segmental ROM and coupling of the foot joints were compared between patients that were operatively treated for a TAF and a control group. Only two previous studies have reported on foot kinematics after the operative treatment of ankle fractures. However, the patient cohorts were heterogeneous and the used Oxford foot model did not include the midfoot as a separate segment [13,14]. Moreover, both studies investigated coupling among the different segments of the foot.

In our study, we observed a general trend towards reduced ROM and joint coupling of the affected foot, particularly the ankle segment and midfoot joints. The patient group presented a mean body mass index corresponding to obesity class I, which is an interesting finding. From a functional viewpoint, it is reasonable to assume that an elevated BMI can be considered as a risk factor for moderate and low impact trauma, as frequently seen in TAFs. Nevertheless, further large-scale population studies are necessary to validate this assumption.

General information provided by the AOFAS score leads to the conclusion that the patients included in the population faced moderate pain and mobility problems at the onset of the investigation. The biomechanical outcome measures quantified here could thus also be (partly) explained by these patient' reported outcome measurements.

When comparing the patients' affected side to the contralateral non-affected side, some differences were obvious. The changes observed in the ankle segment during the loading response and the pre-swing phase can be a result of possible arthritis and arthro-fibrosis of the ankle joint and fibro-adhesions (i.e., muscle adhesions) due to surgery, which all affect ankle joint mobility [7–10]. Additionally, the involvement of pain at the affected foot may also contribute to the reduced ROM seen during the loading response and the pre-swing phase. The reduced ROM during the pre-swing phase may originate from the weakness of the calf muscles on the one hand, but could also be associated with a (mal)adaptive strategy of the patient in order to avoid peak loading in the posterior part of the ankle joint.

The Chopart joint of the affected foot had a significantly reduced ROM in the frontal plane during the loading response. A similar observation has been reported by Eerdekens et al. in patients with ankle osteoarthritis. This tibiotalar stiffness leads to the conclusion that this reduced motion is possibly associated with co-contraction and a more cautious walking strategy [23].

During the pre-swing phase, the MTP 1 joint of the affected foot showed a reduced ROM (mainly dorsal flexion) (Table 3 and Figure 1). This restriction may consequently be caused by muscle adhesions (fibro-adhesions) of the flexor hallucis longus muscle due to posterior plate osteosynthesis. However, from a functional viewpoint, this finding may also highlight a suboptimal usage of the "windlass mechanism" during propulsion, which in turn may affect the physiological joint coupling among the joints of the foot [24].

When comparing the affected foot of the patient group to the control group, a significant reduced transverse plane ankle ROM was quantified during the loading response. The latter observation explains the medium joint coupling observed between ankle inversion– eversion and adduction–abduction. Potential causes for this medium joint coupling may be due to arthro-fibrosis and fibro-adhesions at the posterior aspect of the ankle, the presence of co-contraction of extrinsic foot muscles, or alterations in foot placement during initial contact.

Differences in the ROM were also observed in the ankle segment and Chopart joint during the midstance, terminal stance, and pre-swing phases. Therefore, it can be concluded that there is less plantar flexion during propulsion in these joints (Figure 1) and the hindfoot tends to maintain a more abducted position. A similar observation was reported in the previous study of van Hoeve et al. [13]. Such a reduced plantar flexion was also observed when comparing the affected foot with the unaffected foot, which seems to point towards the presence of weakness of the calf muscles on the one hand or the presence of a (mal)adaptive strategy, avoiding any peak loading in the posterior aspect of the talus and posterior malleolus.

In the current study, small joint coupling was observed in the affected foot. One may hypothesize that this could be caused by perturbed neuromuscular control, proprioception, or the presence of arthro-fibrosis and muscle adhesions at the posterior aspect of the ankle. Despite the fact that these assumptions are realistic and logic, it should be recognized that a similar level of joint coupling was observed in the non-affected side of the patient group. This was an unexpected finding in the study and raises two new hypotheses. The first concerns whether the unaffected limb adopts a (mal)adaptive movement pattern to strive for gait symmetry. This adaptation is also seen in the knee after ACL reconstruction, where kinematic differences between the ACL-reconstructed limb and contralateral unaffected limb decrease over time because of alterations in both limbs [25]. A second hypothesis proposes that this smaller joint coupling was a pre-existing biomechanical phenomenon prior to the ankle trauma and that, together with the increased body mass, it can be considered as a risk factor for the development of ankle fractures. Further research is needed validate to validate or reject this hypothesis.

The postoperative rehabilitation for patients treated for a TAF needs to place emphasis on regaining full ankle joint mobility with passive and active exercises, as well as early protective weight bearing. Full plantar flexion mobility should be highlighted and transferred into the gait pattern, with a focus on the pre-swing phase. Gait training is thereby an important aspect of rehabilitation. Previous studies have concluded that early postoperative mobilization and weight bearing is safe and does not increase the complication rate in patients treated for ankle fractures [26–28]. Combining these aspects with weight reduction could lower the risk of developing post-traumatic osteoarthritis. Patients need to receive a home exercise program so that daily practice is possible and the patient can be autonomous in their treatment.

An important limitation of the current study is the non-standardized period between surgery and gait analysis. Within the patient group, the range was between 8 and 49 months after the operation. The differences in time between both events will influence the collected data because of dissimilarities in the recovery time. In addition, the low case numbers and the minimal study power result in limited practical applications. Another limitation is the condition of barefoot walking when the data were collected. When wearing shoes, the kinematic data may differ from the data during barefoot walking [29]. Lastly, this study only observed patients during walking. Therefore, these outcomes are not representable for more challenging tasks, e.g., running or jumping. It is hypothesized that more complex and challenging tasks may unravel other biomechanical differences than those reported here.
