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Progressive collapsing foot disorder is a complex deformity encompassing pronation and abduction of the midfoot and valgus alignment of the hindfoot, with a flattened medial arch . Every flatfoot case presents some degree of peritalar instability depending on the severity of the disease. In flexible deformities, joint-preserving techniques are currently the preferred approach, including osteotomies and soft tissue repairs or augmentations. The most frequently used osteotomy is the medializing calcaneal osteotomy (MDCO), initially described by Gleich in 1893 and later popularized by Koutsoggianis and described in most review articles ^,, . Dilwyn Evans observed the effect on foot alignment of overshortening the lateral column and thus proposed that the opposite (i.e., a lateral column lengthening) would medialize the heel and recover the convexity of the medial border of the foot . A lateral column lengthening effect is desirable as it has been shown to correct primarily forefoot abduction and hindfoot valgus ^, . The Step cut osteotomy (“Z” osteotomy) has been described and shown to be similar to the Evans osteotomy ^, . Basically, it modifies the Evans osteotomy, starting more proximally and finishing proximally just anterior to the peroneal tubercle, adding to the lengthening effect a rotational effect, correcting the abduction deformity. In two clinical studies, good results have been shown with even faster healing times than the classic Evans ^, . A different osteotomy, the calcaneal “scarf” or inverted Z (IZ) technique was proposed in 2001 by Lowell Weil . This osteotomy consists of a longitudinal central arm (oriented with a 30 degrees plantar declination) from the postero-superior aspect of the calcaneal tuberosity posteriorly to the posterior aspect of the subtalar joint anteriorly, with vertically oriented dorsal-proximal and plantar distal short vertical arms. In this way, this osteotomy lengthens the calcaneus itself and probably achieves a lateral column lengthening effect through tensioning the short and long plantar ligaments and the peroneus longus tendon. Radiological results were shown with a good level of correction of every flatfoot measure, but with 12 % of sural neuritis reported .

Tendon transfers have recently been proposed to aid in the surgical correction of flatfeet cases. A peroneus brevis (PB) to longus (PL) transfer has been used in flexible flatfoot patients to remove a deforming force (abduction of the forefoot) and provide plantar flexion of the first metatarsal. Conti recently showed that a PB to PL transfer could be helpful in the surgical treatment of flatfeet as a coadjuvant treatment . No clinical results have been shown yet. To our knowledge, no biomechanical study has been presented comparing the effect of calcaneal osteotomies in a flatfoot model with or without a PB to PL transfer.

Our objective in this study was to compare, in a cadaveric flatfoot model, different calcaneal osteotomies used for flatfoot surgical correction adding a peroneus brevis to longus transfer, measuring angular changes.

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15 fresh frozen lower leg cadaveric specimens were utilized. All specimens belonged to individuals under 65 years old without previous surgeries or visible pathology. They were thawed at room temperature for 16 h before testing. The specimens were manipulated, stored, and disposed of according to approved protocols.

Every specimen was mounted onto a specific frame designed for this experiment, allowing the specimen to remain vertical to simulate a mid-stance phase of gait. The tibia was fixed to a metallic vertical shaft using five screws, which impeded any motion between the shaft and the specimen. To avoid any medial ankle laxity and restrict the results obtained to changes involving the peritalar joints, the ankle joint was fixed with three screws (4.5 mm) at 90 degrees, leaving the subtalar joint intact to permit normal adaptation of the midtarsal joints. The skin and fat layer of the lateral hindfoot and medial ankle were removed, exposing the talonavicular capsule and spring ligament medially and the peroneal tendons and the region of the calcaneus and cuboid bones laterally. The foot was firmly stabilized to the ground. To achieve this, an axial load was applied to the specimen, and a rough surface was placed under the foot. Below every specimen, a force plate (FP-4000. Bertec Corp. USA) was installed to control the amount of axial load applied ( Fig. 1 ).

General setup for the experiment. A specifically designed frame is seen holding a specimen. Infrared cameras around the testing frame allowed for capturing changes in angular position.

An axial load of 175 N was used for every testing condition (a quarter of body weight) to

preserve the longevity of the cadaveric specimens. The tendons of the tibialis posterior (TP), flexor hallucis longus (FHL), flexor digitorum longus (FDL), peroneus brevis (PB), and peroneus longus (PL) tendons were identified on the proximal stump of every specimen and fixed on to a soft tissue clamp. These tendons were secured with steel ropes to a pulley system to exert a controlled pull using dead weights. The load onto the identified tendons was assigned using published information equivalent to 50 % of the tendon load at the mid-stance phase (TP 60 Newtons (N), FHL 30 N, FDL 15 N, PB 24 N, PL 48 N) . No load was applied to the extensor tendons as they are inactive in the stance phase of gait.

Kinematically, we recorded the 3D spatial alignment of the foot using a Vicon positioning system (Vicon Serie-T, Vicon Motion Systems Ltd. Oxford. UK). To achieve this, four clusters with three reflective markers each were placed on the talus, sustentaculum tali of the calcaneus, dorsal surface of the navicular, and base of the first metatarsal. The spatial position of the clusters was recorded using eight infrared cameras symmetrically positioned around the room that detected (sample rate 50 Hz) the reflective markers’ movement throughout the testing ( Fig. 2 ).

A testing frame is shown, holding a specimen. The frame allowed the application of controlled weight onto the specimen. A pressure plate was installed underneath the specimen (not shown) to compare any change after applying experimental conditions.

Three different conditions were tested for each specimen, i.e., intact specimens, flatfoot specimens, and “repaired” specimens. The anatomical alignment using the 3D spatial positioning of every reflective cluster of the foot was recorded in these three conditions, repeating every measurement 5 times.

For the flatfoot deformity condition, the talonavicular capsule, the tibio-navicular portion of the deltoid ligament, the spring ligament, and the subtalar interosseous ligaments were cut . The tibialis posterior tendon was not pulled to represent insufficiency in this model. Cyclic axial compression of 800 N was then applied to the specimens until talonavicular abduction increased compared to the intact condition, which the motion capture system confirmed.

For the repaired specimens, five different calcaneal osteotomies designed for flatfoot correction were performed. After every osteotomy, a peroneus brevis to longus transfer (PBtoPL) was added to the model, releasing the traction placed onto the PL tendon and transferring it to the PB tendon. The 3D spatial position measurements were recorded through the Vicon system after each osteotomy with and without a PBtoPL transfer. The osteotomies performed included a medial displacement calcaneal osteotomy (MDCO), Evans osteotomy, double osteotomy (MDCO + Evans), Step cut osteotomy (“Z” osteotomy), and calcaneal scarf osteotomy (inverted “Z” osteotomy or IZ). Five specimens were prepared with Evans and MDCO osteotomies, five specimens were prepared with the Z osteotomy and 5 specimens were prepared with the IZ osteotomy. Briefly, the MDCO osteotomy was performed as described by Feuerstein . A 1 cm medial displacement was performed in every specimen. The Evans osteotomy was performed as described by Roche, performing a calcaneal osteotomy parallel to and 1 cm proximal to the calcaneo-cuboid joint with a small oscillating saw, lengthening the lateral column and inserting a 6 mm trapezoidal wood wedge into the osteotomy . We chose a 6 mm wedge due to reports where bigger wedges are associated with lateral column overload, and sizes between 4 and 6 mm would be preferable . We used the same wedge size for the “Z” and the “IZ” osteotomies. The double osteotomy consisted of combining the MDCO and Evans techniques. The “Z” osteotomy was performed as described by Demetracopulos where a distal dorsal vertical calcaneal osteotomy was performed in a similar position as for the Evans osteotomy but only progressing through the dorsal one-third of the bone, adding then a horizontal limb directed posteriorly to a point just distal to the peroneal tubercle, and a short proximal vertical limb exiting plantar, adding a 6 mm trapezoidal wood wedge into the distal osteotomy limb . Finally, the IZ osteotomy was performed as described by Weil where a small vertically oriented dorsal proximal calcaneal osteotomy is performed in a similar position as the MDCO osteotomy but progressing only through one-third of the bone . Then, a horizontal limb is added, aiming distally in a 30-degree inclination plantar, exiting through a short vertically oriented distal plantar arm, which is situated 2 cm anterior to the plantar calcaneal tuberosity. A 6 mm trapezoidal wood wedge was inserted into the proximal vertical osteotomy, allowing lengthening and rotation of the calcaneus ( Fig. 3 ). Every osteotomy was fixed rigidly in place with Kirschner wires supplemented with a 7.0 fully threaded axial screw.

Diagram of the “Z” and “IZ” osteotomies. A calcaneus diagram on the left shows a red dotted line representing a “Z” or step-cut osteotomy. A calcaneus diagram on the right of the picture is shown where a green dotted line represents an “Inverted Z” (IZ) or scarf osteotomy.

After each condition was generated, we analyzed the change in axial talonavicular (TN) and axial talometatarsal (TM) angles to determine the adduction effect of each osteotomy. We also analyzed the change in the coronal talonavicular (CTN) angles to determine the supination effect of each osteotomy. Every measure was obtained and analyzed relative to the uninjured condition (basal). A negative axial TN and TM angle value was assigned to any abduction deformity, and a positive CTN value was assigned to any pronation deformity. A bigger adduction and supination effect would correspond to osteotomies with the highest correction power. Statistical analysis was performed by a statistician using Analysis of Variance (ANOVA) for repeated measures, estimates by mixed models, p-values were one side tail (Dunet´s test), Stata version 18.0. A post hoc power analysis achieved a 0.85 power to detect differences with the numbers available.

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After creating the flatfoot model, a statistically significant abduction and pronation deviation was observed (decrease in axial TN and TM angles and increase in CTN angle, respectively). The axial TN and TM angles decreased by 5.3 and 5.5 degrees, respectively ( Tables 1 and 2 ). The CTN angle increased by 3.2 degrees ( Table 3 ). Variable non-correlated changes were observed in the coronal talocalcaneal or sagittal talometatarsal angles and therefore are not included in this analysis.

Table 1

Talonavicular (TN) angle changes for each condition. TN angle is shown as average, along with the 95 % confidence interval and “p” values. The “adduction effect” refers to the number of degrees of adduction each condition generated. A bigger positive value represents more adduction effect, and a negative value represents an abduction effect. PbtoPL refers to the peroneus brevis to peroneus longus transfer. MDCO refers to medial displacement calcaneal osteotomy. “Z” refers to the step cut or “Z” osteotomy. “IZ” refers to the calcaneal scarf or Inverted Z osteotomy.

		Adduction effect versus flatfoot condition (degrees)	Axial Talonavicular Angle (average in degrees)	Axial Talonavicular angle 95 % Confidence Interval		“p” value against basal	“p” value against flatfoot	“p” value against condition without transfer
Condition				Min	Max
Flatfoot			−5.3	−7.0	−4.2	0.00
PbtoPL		0	−5.3	−7.3	−3.3	0.00	0.99
Evans		15.1	9.8	6.7	12.9	0.00	0.00
Evans+PBtoPL		15.1	9.8	6.7	12.9	0.00	0.00	1.00
Evans+MDCO		13.2	7.9	4.8	11.0	0.00	0.00
Evans+MDCO+PbtoPL		12.9	7.6	4.5	10.8	0.00	0.00	0.89
MDCO		0.7	−4.6	−7.7	−1.4	0.00	0.65
MDCO+PBtoPL		−0.8	−6.1	−9.2	−2.9	0.00	0.63	0.41
“Z”		7.1	1.8	1.3	4.9	0.27	0.00
“Z” + PBtoPL		6.8	1.5	−1.7	4.6	0.36	0.00	0.87
IZ		3.41	−1.89	−5.0	1.2	0.24	0.00
IZ+PBtoPL		3.9	−1.4	−4.6	1.7	0.37	0.02	0.81

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