Josh R. Baxter

Ankle joint mechanics and foot proportions differ between human sprinters and non-sprinters

Recent studies of sprinters and distance runners have suggested that variations in human foot proportions and plantarflexor muscle moment arm correspond to the level of sprint performance or running economy. Less clear, however, is whether differences in muscle moment arm are mediated by altered tendon paths or by variation in the centre of ankle joint rotation. Previous measurements of these differences have relied upon assumed joint centres and measurements of bone geometry made externally, such that they would be affected by the thickness of the overlying soft tissue. Using magnetic resonance imaging, we found that trained sprinters have shorter plantarflexor moment arms (p = 0.011) and longer forefoot bones (p = 0.019) than non-sprinters. The shorter moment arms of sprinters are attributable to differences in the location of the centre of rotation (p < 0.001) rather than to differences in the path of the Achilles tendon. A simple computer model suggests that increasing the ratio of forefoot to rearfoot length permits more plantarflexor muscle work during plantarflexion that occurs at rates expected during the acceleration phase following the sprint start.

Anatomic Ligament Repair Restores Ankle and Syndesmotic Rotational Stability as Much as Syndesmotic Screw Fixation.

Objectives: To compare the external rotation stability of 3 different syndesmotic stabilization techniques in a cadaveric ankle fracture model. Methods: Nondestructive external rotation stresses of 4 N·m were applied to 8 cadaveric limbs using a hydraulic loading frame. Four conditions were tested using a repeated-measures design: intact and 3 repair conditions after a destabilizing ligamentous ankle injury with syndesmotic disruption. The 3 repair conditions were tricortical trans-syndesmotic screw fixation, posterior inferior tibiofibular ligament (PITFL) repair, and combined PITFL and deltoid ligament repair. External rotation of the ankle joint and syndesmosis was measured using a motion capture system and compared for each test condition. Repeated-measures 1-way analyses of variance statistical tests were performed to compare the ankle and syndesmotic rotation findings between the 3 repair conditions and intact condition. Results: Rotational ankle stability was not fully restored by any of the 3 repair constructs. The intact ankle joint externally rotated approximately half as many degrees as the 3 repair conditions (intact: 10.9; trans-syndesmotic screw: 17.0; PITFL: 21.4; and PITFL/deltoid: 15.6). The intact condition also demonstrated significantly fewer degrees of syndesmotic rotation than the repair constructs (intact 2.4; trans-syndesmotic screw 5.2; PITFL 8.5; and PITFL/deltoid 6.9). Each of the repair conditions resulted in an externally rotated fibula when no loads were applied. The ligamentous repairs externally rotated the fibula twice as much as the trans-syndesmotic screw (P < 0.016). Conclusions: We found that combined repair of the PITFL and deltoid ligament restores an equivalent amount of ankle and syndesmotic rotational stability when compared to trans-syndesmotic screw fixation. Based on our findings, ligamentous repair can potentially be a viable treatment alternative in unstable ankle fracture patients with syndesmotic disruption. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Plantar flexor moment arm and muscle volume predict torque-generating capacity in young men

Muscle volume is known to correlate with maximal joint torque in humans, but the role of muscle moment arm in determining maximal torque is less clear. Moderate correlations have been reported between maximal isometric knee extensor torque and knee extensor moment arm, but no such observations have been made for the ankle joint. It has been suggested that smaller muscle moment arms may enhance force generation at high rates of joint rotation, but this has not yet been observed for ankle muscles in vivo. The purpose of the present study was to correlate plantar flexor moment arm and plantar flexor muscle volume with maximal plantar flexor torque measured at different rates of plantar flexion. Magnetic resonance imaging was used to quantify the plantar flexor moment arm and muscle volume of the posterior compartment in 20 healthy young men. Maximal plantar flexor torque was measured isometrically and at three plantar flexion speeds using an isokinetic dynamometer. Plantar flexor torque was significantly correlated with muscle volume (0.222 < R(2) < 0.322) and with muscle moment arm at each speed (0.323 < R(2) < 0.494). While muscle volume was strongly correlated with body mass and stature, moment arm was not. The slope of the torque-moment arm regression line decreased as the rate of joint rotation increased, indicating that subjects with small moment arms experienced smaller reductions in torque at high speeds. The findings of this study suggest that plantar flexor moment arm is a determinant of joint strength that is at least as important as muscle size.

Limited-Open Achilles Tendon Repair Using Locking Sutures Versus Nonlocking Sutures An In Vitro Model

Background: Several limited-open Achilles tendon repair techniques that use locking or nonlocking sutures have been developed, but direct comparisons of in vitro mechanical properties have not yet been reported in the literature. It was our hypothesis that loads applied to the repaired Achilles tendon would be better resisted by limited-open techniques that use locking stitches compared with limited-open repairs that use nonlocking stitches. Methods: The Achilles tendons of 31 fresh-frozen cadaver lower limbs were incised 4 cm proximal to the calcaneal insertion. Tendons were then repaired using 1 of 2 limited-open Achilles tendon repair tools, one using 3 nonlocking sutures and the other using a combination of locking and nonlocking sutures. Repaired specimens were cycled to 1000 cycles from 20 to 100 N and from 20 to 190 N followed by a single load to failure test. Nonparametric analyses were performed to compare the number of cycles to gapping and total load to failure between the 2 repair techniques. Results: During cyclic loading, more cycles occurred prior to detection of 2-mm and 9.5-mm gaps in the locking suture construct compared with the nonlocking suture construct (P = .012 and P = .005, respectively). There was no difference in the number of cycles to a gap of 5 mm (P = .053). The locking suture construct also resisted a significantly greater load to failure compared with the nonlocking suture construct (P < .001; median 385.0 and 299.6 N, respectively). Conclusion: Limited-open repair techniques using locking sutures provided greater construct strength under both cyclic and ultimate loads compared with a repair technique that used only nonlocking sutures. Clinical Relevance: Limited-open Achilles tendon repairs using locking sutures are better able to resist forces simulating early accelerated rehabilitation than repairs using nonlocking sutures.

Lateral Column Lengthening Corrects Hindfoot Valgus in a Cadaveric Flatfoot Model

Background: Adult-acquired flatfoot deformity requires a complex operative plan that often utilizes a number of procedures to correct deformity, at both the hindfoot and midfoot. A lateral column lengthening procedure is typically performed to correct abduction deformity across the talonavicular joint; however its effect on hindfoot alignment is not well understood, and overcorrecting the hindfoot deformity can lead to pain and revision surgeries. Therefore, understanding the effect of lateral column lengthening on hindfoot alignment is important for operative planning. The aim of this study was to demonstrate the effect of a lateral column lengthening osteotomy on hindfoot valgus in a simulated flatfoot model. Methods: A flatfoot deformity was created in 12 lower limb cadaveric specimens. A step-cut lateral column lengthening osteotomy was performed and changes in hindfoot and midfoot alignment were measured using a motion capture system. Results: The lateral column lengthening procedure corrected 60% of the hindfoot valgus deformity (P < .001). In addition, the abduction deformity at the midfoot was completely corrected with the procedure (P < .001). Conclusion: Our findings suggest that the lateral column lengthening procedure corrects hindfoot valgus, in addition to midfoot abduction, in a flatfoot deformity. Clinical Relevance: Understanding the relative contribution of each operative procedure to the overall deformity correction in flatfoot reconstruction will allow surgeons to accurately correct the foot into a plantigrade position while minimizing the risk of overcorrection.

Crossed-Screws Provide Greater Tarsometatarsal Fusion Stability Compared to Compression Plates

Background. Hallux valgus is a common deformity that is often treated with a fusion of the first tarsometatarsal (TMT) joint. Crossed-screws are currently the accepted standard but advances in plate systems present opportunities for improved clinical outcomes; however, in vitro testing should be performed prior to clinical implementation. The purpose of this study was to determine whether a locking plate with surgeon-mediated compression provides similar fusion stability compared to crossed-screws and if bone density or joint size are related to construct success. Methods. Ten matched-pair cadaveric specimens received first TMT fusions with either crossed-screws or a compression plate and were loaded for 1000 cycles to assess the amount of joint motion measured as plantar gapping. Bone density was quantified using computed tomography images of each specimen, and joint height was measured with calipers. Results. Crossed-screws provided 3 times greater resistance to plantar gapping compared to compression plates after 1000 cycles. Bone density and joint size did not affect resistance to plantar gapping for either construct. Conclusion. Lag screws or a plantarly applied plate are needed to maximize TMT fusion stability prior to osseous union. Dorsomedially applied plates are also effective when paired with a lag screw placed across the TMT joint. These constructs do not appear to depend on bone density or joint size, suggesting that patients with osteoporosis are viable candidates. Clinical Relevance. The results of this study suggest that traditional, lagged cross-screws provide greater stability to that of a dorsally place compression plate and may lead to better rates of union. Levels of Evidence: Therapeutic, Level V: Cadaveric Study

Three-Dimensional Analysis of Fibular Motion After Fixation of Syndesmotic Injuries With a Screw or Suture-Button Construct

Background: Suture-button constructs are an alternative to screw fixation for syndesmotic injuries, and proponents advocate that suture-button constructs may allow physiological motion of the syndesmosis. Recent biomechanical data suggest that fibular instability with syndesmotic injuries is greatest in the sagittal plane, but the design of a suture-button construct, being a rope and 2 retention washers, is most effective along the axis of the rope (in the coronal plane). Some studies report that suture-button constructs are able to constrain fibular motion in the coronal plane, but the ability of a tightrope to constrain sagittal fibular motion is unknown. The purpose of this study was to assess fibular motion in response to an external rotation stress test in a syndesmotic injury model after fixation with a screw or suture-button constructs. Methods: Eleven fresh-frozen cadaver whole legs with intact tibia-fibula articulations were secured to a custom fixture. Fibular motion (coronal, sagittal, and rotational planes) in response to a 6.5-Nm external rotation moment applied to the foot was recorded with fluoroscopy and a high-resolution motion capture system. Measures were taken for the following syndesmotic conditions: intact, complete lateral injury, complete lateral and deltoid injury, repair with a tetracortical 4.0-mm screw, and repair with a suture button construct (Tightrope; Arthrex, Naples, FL) aimed from the lateral fibula to the anterior medial malleolus. Results: The suture-button construct allowed significantly more sagittal plane motion than the syndesmotic screw. Measurements acquired with mortise imaging did not detect differences between the intact, lateral injury, and 2 repair conditions. External rotation of the fibula was significantly increased in both injury conditions and was not restored to intact levels with the screw or the suture-button construct. Conclusion: A single suture-button placed from the lateral fibula to the anterior medial malleolus was unable to replicate the motion observed in the intact specimen when subjected to an external rotation stress test and allowed significantly more posterior motion of the fibula than when fixed with a screw in simulated highly unstable injuries. Clinical Relevance: Fixation of a syndesmotic injury with a single suture-button construct did not restore physiological fibular motion, which may have implications for postoperative care and clinical outcomes.

Reconstruction of the medial talonavicular joint in simulated flatfoot deformity.

Background: Reconstructing the ligamentous constraints of the medial arch associated with adult acquired flatfoot deformity remains a challenge. The purpose of this study was to test the efficacy of several reconstruction techniques of the medial arch. We hypothesized that an anatomic reconstruction of the spring ligament complex would correct the deformity better than other techniques tested. Methods: Three reconstructions of the medial support structures were performed on each specimen to recreate the different lines of action and insertions of the medial ligamentous complex in 12 specimens with a simulated flatfoot deformity. Talonavicular and tibiocalcaneal (hindfoot) orientations were measured in the axial, sagittal, and coronal planes in the intact, flatfoot, and reconstructed conditions. Results: While each reconstruction technique corrected the deformity (P < .05), proximal fixation of the graft corrected the greatest amount of talonavicular deformity while also correcting hindfoot valgus (P < .05). Conclusion: The fixation points and lines of action of a medial arch reconstruction have important implications on deformity correction in a flatfoot model. Despite its fidelity to the native structure, the anatomic spring ligament reconstruction provided the least amount of correction. These findings suggest that other ligamentous structures of the medial arch are critical in supporting the midfoot. Clinical Relevance: Reconstruction of the ligamentous supports of the medial arch might be able to correct substantial amounts of deformity without osseous procedures like calcaneal osteotomies or midfoot fusions.

Graft Shape Affects Midfoot Correction and Forefoot Loading Mechanics in Lateral Column Lengthening Osteotomies

Background: Adult acquired flatfoot deformity is characterized by midfoot abduction and collapse of the medial longitudinal arch. Lateral column lengthening osteotomies primarily correct the abduction deformity, but the effects of graft shape on deformity correction and forefoot loading are unclear. Therefore, the purpose of this study was to demonstrate the effect of graft shape and taper on deformity correction and forefoot loading mechanics in a cadaveric flatfoot model. Methods: Flatfoot deformity was simulated in 18 cadaveric specimens. A lateral column lengthening osteotomy was performed using a triangular, trapezoidal, and rectangular graft for each specimen. During each testing condition, talonavicular joint angles and forefoot plantar pressures were measured. Results: Each graft shape corrected abduction and dorsiflexion deformity at the talonavicular joint. Coronal plane correction was affected by graft shape, and the less tapered trapezoidal and rectangular grafts overloaded the lateral forefoot compared to the intact condition. The more tapered triangular graft did not cause a lateral shift in forefoot pressures. Forefoot plantar pressures were strongly correlated with talonavicular abduction correction (R2 = .473, P < .001). Conclusion: Graft shape had no effect on the correction of talonavicular abduction or dorsiflexion but did influence coronal plane motion and forefoot loading mechanics. Also, overcorrecting the abduction deformity was predictive of increased lateral plantar pressures. Clinical Relevance: Although overcorrection of the abduction deformity at the midfoot remains the primary determinant of lateral forefoot overload, utilizing a graft with a larger taper may lower the incidence of lateralized forefoot pressure following correction.

Effect of Complete Syndesmotic Disruption and Deltoid Injuries and Different Reduction Methods on Ankle Joint Contact Mechanics

Background: Syndesmotic injuries can be associated with poor patient outcomes and posttraumatic ankle arthritis, particularly in the case of malreduction. However, ankle joint contact mechanics following a syndesmotic injury and reduction remains poorly understood. The purpose of this study was to characterize the effects of a syndesmotic injury and reduction techniques on ankle joint contact mechanics in a biomechanical model. Methods: Ten cadaveric whole lower leg specimens with undisturbed proximal tibiofibular joints were prepared and tested in this study. Contact area, contact force, and peak contact pressure were measured in the ankle joint during simulated standing in the intact, injured, and 3 reduction conditions: screw fixation with a clamp, screw fixation without a clamp (thumb technique), and a suture-button construct. Differences in these ankle contact parameters were detected between conditions using repeated-measures analysis of variance. Results: Syndesmotic disruption decreased tibial plafond contact area and force. Syndesmotic reduction did not restore ankle loading mechanics to values measured in the intact condition. Reduction with the thumb technique was able to restore significantly more joint contact area and force than the reduction clamp or suture-button construct. Conclusion: Syndesmotic disruption decreased joint contact area and force. Although the thumb technique performed significantly better than the reduction clamp and suture-button construct, syndesmotic reduction did not restore contact mechanics to intact levels. Clinical Relevance: Decreased contact area and force with disruption imply that other structures are likely receiving more loads (eg, medial and lateral gutters), which may have clinical implications such as the development of posttraumatic arthritis.