Introduction

Progressive collapsing foot deformity (PCFD), also known as “adult-acquired flatfoot deformity” (AAFFD), is a complex foot deformity consisting of multiple components: (1) peritalar subluxation resulting in foot deviation in various planes (e.g., hindfoot valgus, talar plantarflexion, forefoot supination); (2) abduction deformity at the level of the midfoot; and (3) forefoot varus with the first ray elevated above the fifth metatarsal [1, 2]. The etiology is still unclear, but generally involves soft tissue degeneration associated with dysfunction of the tibialis posterior tendon (PTT) [3, 4].

If surgery is required, joint-preserving procedures, such as a medial displacement calcaneal osteotomy (MDCO), are frequently performed. Calcaneal osteotomies as part of a bony reconstruction address hindfoot valgus and may also impact midfoot abduction [5,6,7]. The MDCO, first described by Gleich in 1893 [8], involves a medial translation of the entire tuber calcanei. As the lever arm of the Achilles tendon changes, such an osteotomy may have a significant effect on foot function.

Several studies assessed the impact of MDCO on foot biomechanics. Nevertheless, the understanding of MDCO on foot function is currently limited. Therefore, the aims of this systematic review are: (1) to perform a systematic literature search on studies assessing the impact of MDCO on foot biomechanics; (2) to provide an overview of the current knowledge on the effect of MDCO on foot biomechanics; and (3) to grade these studies according to their methodological quality.

Materials and methods

Search strategy and study selection

The original protocol for this systematic literature review was registered on PROSPERO, the international prospective register of systematic reviews (CRD42022270180), after performing a search on the CRD database to find out whether this review was already registered. The electronic MEDLINE database via PubMed and Scopus were systematically searched. The search was performed on the 5th of July 2021, with a final update on the 25th of January 2023. The following search algorithm was used: (calcan* [ALL] AND osteotomy [ALL]) OR (sliding AND calcan*) OR (calcaneus [MeSh] AND osteotomy [MeSh]). There were no limitations on the type of journal or article publication date. Only publications in English were included. The article bibliographies were also reviewed. Bidirectional citation search was used including backward and forward citation search methods [9]. The systematic literature search was conducted independently by three authors (GM, CS and EM) according to the PRISMA (Preferred Reporting Items for Systematic Review and Meta-Analysis) guidelines [10]. Studies were included if they were original research reporting data on biomechanical changes following a MDCO. Exclusion criteria included review articles, case studies, and studies not written in English. The study selection process was performed independently by three observers (EM, GM, CS). In case of a disagreement, a final decision was made by group consensus.

Data extraction and quality assessment

The following data from the included studies were extracted: type of study, number of cadaveric specimens / number of biomechanical models / number of feet, age, biomechanical testing method (if appropriate), and the impact on foot biomechanics as primary outcome parameters. In addition, proposed therapeutic and/or surgical consequences were collected as secondary outcome parameters. Data extraction was independently performed by two review authors (GM and EM). The methodological quality of the studies including cadavers was assessed using the QUACS scale (Quality Appraisal for Cadaveric Studies; Table 1) [11]. Two reviewers (GM and CS) independently examined each included study using the checklist consisting of 13 items and reported scores as percentages. These scores were pooled and reported as an average. Clinical studies were graded using the modified Coleman score (Table 2) [12]. The modified Coleman score was applied by two independent reviewers (GM and CS). Each study was assessed for study size, follow-up time, percentage of patients with follow-up, number of interventions, study type, diagnostic certainty, description of surgical method/rehabilitation, outcome criteria/assessment process, and patient selection process.

Table 1 Items and scoring criteria of the QUACS scale
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Table 2 Modified coleman methodology score
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Statistical analysis

The data were processed descriptively; therefore, no meta-analysis was performed. Patient demographic characteristics (number of patients/ feet, patient age and sex) were summarized. Weighted median scores were calculated for the modified Coleman and QUACS scores. Data analysis was performed using IBM SPSS Statistics Version 26.0 (IBM Corp., Armonk, NY, USA).

Results

The initial database screening resulted in 1,900 studies. After removing all duplicates and screening of titles and abstracts, 157 studies were eligible for full-text review. After exclusion of 130 studies according to our exclusion criteria (review articles, incomplete data set, no information on foot biomechanics), 27 studies were ultimately included in the final analysis. The selection process was performed according to “Preferred Reporting Items for Systematic Review and Meta-Analyses” (PRISMA) and is shown in Fig. 1 [10].

Fig. 1
figure 1

Flowchart depicting the strategy used to select relevant studies

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Study characteristics

The 27 included studies were published between 1995 and 2020 and contained 18 cadaveric studies, 7 studies using biomechanical models, and 2 clinical studies (Table 3). The 18 cadaveric studies included 185 cadavers (188 feet) with a pooled mean age of 65 years (range 43–96 years; age defined in 6 studies), comprised of 13 (50%) female and 13 (50%) male patients (gender defined in 4 studies). Within the 7 studies using biomechanical models, the models were based either on cadavers or MRI / CT scan datasets (including 2 Finite-Element-models). The 2 clinical studies evaluated a total of 93 patients (96 feet) with a pooled mean age of 58 years (range 43–79), comprised of 12 (13%) male and 81 (87%) female patients. Studies including cadavers had a pooled mean QUACS score of 65% (range 46–92%). The pooled mean Coleman score of the clinical studies was 58 (range 56–65) points (Table 3). Within the included studies, 4 were financially supported by different grants.

Table 3 Study type characteristics, modified Coleman methodology score and QUACS Score of 27 studies addressing biomechanics of medial displacement calcaneal osteotomy
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Impact on the plantar fascia

Within the included studies, 6 assessed the impact of MDCO on the plantar fascia (Table 4). Thordarson et al. showed in a cadaveric study that a transection of the plantar fascia does not affect the corrective potential of MDCO [13]. Iaquinto et al. demonstrated an 80% increase of plantar fascia strain in AAFFD patients, with a significant decrease following MDCO [14]. Similar findings were evident in the studies published by Smith et al. and Spratley et al. [15,16,17]. Horton and colleagues demonstrated that a 1 cm MDCO resulted in an average of 1.2 ± 0.5 mm shortening of the plantar fascia. This finding was confirmed by Sung et al. [18].

Table 4 Studies reporting of impact of a MDCO on the plantar fascia
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Impact on the medial longitudinal arch

The impact of MDCO on the medial longitudinal arch was evaluated in 9 studies (Table 5). Otis et al. analyzed the length of the spring ligament under a load of 100 N in a cadaver study. The length was comparable before and after MDCO [19]. Sung and colleagues confirmed these results in their cadaveric study [18]. Arangio et al. used a 3D biomechanical multisegmental model to analyze the effect of MDCO on the excess forces in the medial longitudinal arch. They demonstrated that MDCO with 10 mm of displacement results in a substantial shift of excess forces toward the lateral side and hereby decreases the load on the medial arch [20]. Hadfield and colleagues confirmed this finding in a cadaver study [21]. In a further study they also showed a 0.5 cm superior translation of the osteotomy to result in a greater load reduction of the first and second metatarsals [22]. Spratley et al. also showed a change in plantar force distribution with a reduce in load of the medial forefoot [17]. Smith et al. proved with patient specific computational models a decrease of plantar fascia, spring ligament, and deltoid ligament sprain loads [15]. Zanolli et al. demonstrated in a cadaveric study that an additional reconstruction of the spring ligament does not change the effect of MDCO [23]. Resnick et al. analyzed the deltoid ligament force under axial loading. Different clinical and surgical conditions were simulated. A decreased force could be detected when MDCO was combined with a triple arthrodesis [24].

Table 5 Studies reporting of impact of a MDCO on the medial longitudinal arch
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Impact on joints pressure

The impact of MDCO on hind- and midfoot joint pressures was analyzed in 10 studies (Table 6). Fairbank and colleagues analyzed 11 fresh frozen cadaver legs with an axial load of 700 N which quantified the tibiotalar joint contact characteristics. The flatfoot model was associated with a shift of ankle joint pressure laterally and a quantitative alteration of the contact area compared to the normal ankle. MDCO altered the lateral contact pressure. The authors therefore concluded that MDCO might be a useful alternative to the tibiotalar arthrodesis in cases of early tibiotalar arthritis secondary to varus or valgus hindfoot deformity [25]. Havenhill et al. demonstrated similar results comparing an UCBL orthosis and MDCO in flatfoot cadavers [26]. Davitt and colleagues measured the pressure in the tibiotalar and subtalar joints in a cadaveric study after MDCO. Following a 1 cm MDCO, the average center of force shifted medially in both joints [27]. Steffensmeier and colleagues demonstrated in their cadaveric study that a 1 cm MDCO results in a shift of the pressure distribution 1.58 mm medially in the tibiotalar joint. However, the differences in contact area, mean contact stress, and maximum stress in the tibiotalar joint were not different after MDCO. Similar to the study by Davitt and colleagues [27], in this study, a slight anteromedial shift of the subtalar joint center of pressure was measured [28]. Patrick and colleagues also analyzed subtalar joint force, contact area, and peak contact pressure before and after 1 cm MDCO in 4 cadaveric specimens. After MDCO, there was a slight decrease of subtalar joint force from 211.4 N to 168.8 N, a decrease of contact area from 3.5cm2 to 3.1cm2, and a decrease of peak contact pressure from 1810 kPa to 1276 kPa. However, all observed changes were not statistically significant [29]. Arangio et al. demonstrated that pathologically increased force in the talo-navicular joint in a flatfoot model almost normalizes after MDCO. They recommended a MDCO in case of a dysfunction of the PTT [30]. Malik and colleagues confirmed these results and recommended a MDCO as an adjunct procedure in talonavicular arthrodesis for patients with a high risk of nonunion [31]. Iaquinto et al. and Spratley et al. proved in a biomechanical model a normalization of joint pressure in the calcaneo-cuboid joint after MDCO [14, 17]. Scott and colleagues and Smith and colleagues examined the effects of MDCO on joint pressures in the hind- and midfoot combined with a lateral column osteotomy [15, 32]. Scott et al. proved with their cadaveric study an increase of lateral forefoot pressures after lateral column lengthening procedures. The addition of MDCO did not lead to an alteration of plantar pressures [32]. Smith and colleagues demonstrated in 6 biomechanical models that the combination of a MDCO with an Evans Osteotomy and a MDCO with a Z-Osteotomy show the greatest amount of correction for both forefoot abduction and hindfoot valgus in a flatfoot model [15].

Table 6 Studies reporting of impact of a MDCO on joints pressure
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Impact on Achilles tendon

The impact of MDCO on the Achilles tendon was analyzed in 5 of the included studies (Table 7). Hadfield and colleagues analyzed the effect of a 1 cm MDCO on Achilles tendon length and plantar foot pressures in 14 cadaveric specimens. The length of the Achilles tendon remained the same after the osteotomy [21]. The same research group analyzed the effect of superior translation of the tuber calcanei with the same parameters. The addition of a 5 mm superior translation did not lead to Achilles tendon lengthening [22]. Nyska and colleagues established an experimental cadaveric AAFFD model by releasing the PTT, spring ligament, and plantar fascia. Applying axial load with a range between 700 and 1400 N substantially aggravated the deformity as confirmed radiographically. Adding a 1 cm MDCO reduced the arch-flattening effect of the Achilles tendon [33]. Arangio et al. proved with their biomechanical model a decreased force provided by the Achilles, flexor hallucis longus (FHL), and flexor digitorum longus (FDL) tendons in AAFFD. Adding a MDCO decreased the force exerted by the Achilles tendon while increasing the force exerted by the peroneus brevis and longus tendons [30]. Sung and colleagues loaded 13 cadaver specimens in a custom testing apparatus. Measurement of PTT force required to achieve early heel rise in an intact foot and were compared to specimens following MDCO and posterior distraction osteotomy. After MDCO, the Achilles force required to achieve the heel rise decreased [18].

Table 7 Studies reporting of impact of a MDCO on Achilles tendon
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Impact on gait pattern

The impact on gait pattern was analyzed in 3 studies (Table 8). Marks and colleagues analyzed gait parameters as well as radiographic alignment in 14 patients with MDCO and six patients with lateral lengthening osteotomy of the calcaneus. In both groups, a significant improvement of all gait parameters was observed. The MDCO group demonstrated improved first ray plantarflexion, while the lateral lengthening group presented with better heel inversion [34]. Michelson and colleagues examined the alteration in ankle motion after MDCO. At maximal dorsiflexion, internal rotation and varus motion increased. There was no significant difference in plantar flexion. Therefore, they hypothesized that a MDCO may predispose to premature ankle arthritis as a consequence of the altered ankle mechanics [35]. In a clinical study by Schuh et al., the authors examined 75 feet of 73 patients with posterior tibial tendon dysfunction (PTTD) stage II who underwent an FDL tendon transfer and MDCO. Plantar pressure distribution and American Orthopaedic Foot and Ankle Society (AOFAS) score were analyzed 48 months after surgery. The authors saw statistically significant correlations between the AOFAS score and loading parameters of the medial midfoot [36].

Table 8 Studies reporting of impact of a MDCO on gait parameter
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The main biomechanical consequences of the MDCO are summarized in Table 9.

Table 9 Main biomechanical consequences of the MDCO
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Discussion

27 studies assessing the biomechanical impact of MDCO on foot function were included in the present systematic review. We found evidence that MDCO impacts function of the plantar fascia, integrity of the medial longitudinal arch, alters peritalar joint pressures, and has an influence on Achilles tendon function. Consequently, specific gait pattern parameters were found to be impacted by MDCO.

Interestingly, our analysis showed an increase of plantar fascia strain in the case of AAFFD with a drop following MDCO [14,15,16,17]. In addition, Sung et al. reported a decrease of plantar fascia length after MDCO [18]. Another study showed no impact of a plantar fascia release on the corrective potential following MDCO [13]. Therefore, an additional release of the plantar fascia may not be effective in cases where MDCO is performed.

MDCO does not change spring ligament length, while spring ligament reconstruction may not impact the expected outcome following MDCO [18, 19, 23]. However, a substantial shift of excess forces towards the lateral aspect of the longitudinal arch was found in several studies [17, 20,21,22, 30], reflecting a high strength of evidence. Resnick et al. additionally analyzed deltoid ligament force under axial load and concluded that MDCO in combination with a triple arthrodesis may be a viable treatment option when the hindfoot cannot be positioned properly following fusion procedures [24]. MDCO may not only correct hindfoot valgus deformity, but also offload the medial aspect of the foot and therefore can serve as a solution after hindfoot fusion procedures with non-union.

The studies showed a shift of the average center of force medially in the tibiotalar and subtalar joints following MDCO [27, 28]. Additionally, an overall decrease of subtalar and tibiotalar joint forces, contact area, and peak contact pressures were evident in several studies [29, 30]. These aspects of impact on joint pressure show a high strength of evidence. MDCO can therefore be recommended in case of a tibialis posterior tendon dysfunction or as an additional procedure in talonavicular arthrodesis for patients with a high risk of non-union [30, 31]. In addition, Iaquinto et al. and Spratley et al. showed a normalization of increased joint pressures in the calcaneo-cuboid joint after MDCO [14, 17].

Of note, studies have shown MDCO to not impact Achilles tendon length [21, 22], while a reduction of the arch-flattening effect of the Achilles tendon was evident in two other studies [30, 33]. Additionally, a decrease in force at the level of the Achilles tendon was required to achieve heel rise position after MDCO [18]. Consequently, an additional lengthening of the Achilles tendon when performing MDCO for PCFD reconstruction may not be required in most patients.

Specific gait pattern parameters showed significant improvement after MDCO [34]. However, in a clinical study, Schuh et al. showed that MDCO impairs function of the lesser toes during the stance phase of walking, with an increased load of the forefoot [36]. In addition, Michelson and colleagues hypothesized that MDCO may predispose to premature ankle arthritis as a consequence of altered ankle mechanics [35]. The studies evaluating gait parameters showed a moderate strength of evidence, showing the need for further research in this field. The negative effects on gait pattern parameters must be considered when MDCO is performed. Afterall, the indication for MDCO must be thoroughly evaluated to avoid these postoperative complications. Weightbearing computed tomography as a newer preoperative diagnostic tool, which presents the possibility of imaging in the physiological standing position to assess for forefoot and hindfoot alignment, could reduce the risk of negative effects of MDCO in the future [37,38,39,40,41,42,43].

Within the included studies, 18 (66,67%) make statements about the predictive value or operative consequences of the biomechanical findings, showing that knowledge about the biomechanical consequences of MDCO is important in understanding the effects of the osteotomy on the entire foot.

The presented study has some limitations. First, our findings are limited by the quality of the included studies. However, this is an inevitable limitation of systematic literature reviews and meta-analyses in general. For example, most of the included studies failed to report the educational level of the researchers. Also, most studies did not mention whether the observations were performed by one or more researchers, a possible source of bias. Second, only studies written in English were considered, excluding valuable contributions written in other languages.

Conclusion

A thorough knowledge of how MDCO impacts foot function is key in properly understanding the postoperative effects of this commonly performed procedure. There is evidence that MDCO effects the plantar fascia, medial longitudinal arch, peritalar joint pressures, Achilles tendon, and consequently specific gait pattern parameters. Future research should consider newer diagnostic tools including weightbearing computed tomography.