Posterior cruciate ligament deficiency (PCLD) leads to structural and proprioceptive impairments of the knee, affecting the performance of daily activities including obstacle-crossing. Therefore, identifying the biomechanical deficits and/or strategies during this motor task would be helpful for rehabilitative and clinical management of such patients. A safe and successful obstacle-crossing requires stability of the body and sufficient foot clearance of the swing limb. Patients with PCLD may face demands different from normal when negotiating obstacles of different heights. The objective of this study was thus to identify the biomechanical deviations/strategies of the lower limbs in unilateral PCLD during obstacle-crossing using motion analysis techniques.

Twelve patients with unilateral PCLD and twelve healthy controls participated in the current study with informed written consent. They were asked to walk and cross obstacles of heights of 10%, 20% and 30% of their leg lengths at self-selected speeds. The PCLD group was asked to cross the obstacles with each of the affected and unaffected limb as the leading limb, denoted as PCLD-A and PCLD-U, respectively. The kinematic and kinetic data were measured with a 7-camera motion analysis system (Vicon, Oxford Metrics, U.K.) and two force plates (AMTI, U.S.A.). The angles of the stance and swing limbs (crossing angles) and the moments of the stance limbs (crossing moments) for each joint in the sagittal plane when the leading limb was above the obstacle were calculated for statistical analysis. A 3 by 2, 2-way mixed-model analysis of variance with one between-subject factor (PCLD-A vs. Control, and PCLD-U vs. Control) and one within-subject factor (obstacle height) was performed (α=0.05). Paired t-test was used to compare the variables between PCLD-A and PCLD-U (α=0.05). SAS version 9.2 was used for all statistical analysis.

When the leading toe was above the obstacle, the PCLD group showed significantly greater hip flexion in the swing limb but decreased dorsiflexion in the stance limb, both in PCLD-A and PCLD-U (P<0.05). Greater knee flexion and greater ankle dorsiflexion were found in the leading limb in PCLD-A (P<0.05). Meanwhile, the PCLD group showed significantly decreased ankle plantarflexor but increased knee extensor crossing moments in the stance limb compared with the Control (P<0.05). None of the calculated variables were found to be significantly different between PCLD-A and PCLD-U (P>0.05).

When crossing the obstacle, patients with PCLD reduced ankle plantarflexor moments that were mainly produced by the gastrocnemius. This may help reduce the posterior instability of the affected knee. Greater knee extensor crossing moments may also help reduce the posterior instability of the standing knee when the leading toe was above the obstacle.

The changed joint kinetics as a result of PCLD were not only seen on the affected side but also on the unaffected side during obstacle-crossing. This symmetrical pattern may be necessary in performing functional activities that may require either the affected side or the unaffected side leading. These results suggest that rehabilitative intervention, including muscular strengthening, on both affected and unaffected sides are necessary in patients with unilateral PCLD.

Knee ligament injury is one of the most frequent sport injuries and ligament reconstruction has been used to restore the structural stability of the joint. Cycling exercises have been shown to be safe for anterior cruciate ligament (ACL) reconstruction and are thus often prescribed in the rehabilitation of patients after ligament reconstruction. However, whether it is safe for posterior cruciate ligament (PCL) reconstruction remains unclear. Considering the structural roles of the PCL, backward cycling may be more suitable for rehabilitation in PCL reconstruction. However, no study has documented the differences in the effects on the knee kinematics between forward and backward pedaling. Therefore, the current study aimed to measure and compare the arthrokinematics of the tibiofemoral joint between forward and backward pedaling using a biplane fluoroscope-to- computed tomography (CT) registration method.

Eight healthy young adults participated in the current study with informed written consent. Each subject performed forward and backward pedaling with an average resistance of 20 Nm, while the motion of the left knee was monitored simultaneously by a biplane fluoroscope (ALLURA XPER FD, Philips) at 30 fps and a 14-camera stereophotogrammetry system (Vicon, OMG, UK) at 120 Hz. Before the motion experiment, the knee was CT and magnetic resonance scanned, which enabled the reconstruction of the bones and articular cartilage. The bone models were registered to the fluoroscopic images using a volumetric model-based fluoroscopy-to-CT registration method, giving the 3-D poses of the bones. The bone poses were then used to calculate the rigid-body kinematics of the joint and the arthrokinematics of the articular cartilage. In this study, the top dead center of the crank was defined as 0° so forward pedaling sequence would begin from 0° to 360°.

Compared with forward pedaling, for crank angles from 0° to 180°, backward pedaling showed significantly more tibial external rotation. Moreover, both the joint center and contact positions in the lateral compartment were more anterior while the contact positions in the medial compartment was more posterior, during backward pedaling. For crank angles from 180° to 360°, the above-observed phenomena were generally reversed, except for the anterior-posterior component of the contact positions in the medial compartment.

Forward and backward pedaling displayed significant differences in the internal/external rotations while the rotations in the sagittal and frontal planes were similar. Compared with forward cycling, the greater tibial external rotation for crank angles from 0° to 180° during backward pedaling appeared to be the main reason for the more anterior contact positions in the lateral compartment and more posterior contact positions in the medial compartment.

Even though knee angular motions during forward and backward pedaling were largely similar in the sagittal and frontal planes, significant differences existed in the other components with different contact patterns. The current results suggest that different pedaling direction may be used in rehabilitation programs for better treatment outcome in future clinical applications.

Subtalar arthrodesis known as talocalcaneal fusion is an end-stage treatment for adult hind foot pathologies. The goal of the arthrodesis is to restrict the relative motion between bones of the subtalar joints, aiming to reduce pain and improve function for the patient. However, the change of the subtalar structures through the fusion is considered a disturbance to the joint biomechanics, which have been suggested to affect the biomechanics of the adjacent joints. However, no quantitative data are available to document this phenomenon. The purpose of the current study was to quantify the effects of subtalar arthrodesis on the laxity and stiffness of the talocrural joint in vitro using a robot-based joint testing system (RJTS) during anterioposterior (A/P) drawer test.

Six fresh frozen ankle specimens were used in this study. The lateral tissues of the specimens were removed but the anterior and posterior talofibular ligaments and calcaneofibular ligament were kept intact. A/P drawer tests were performed on each of the specimens at neutral position, 5° and 10° of dorsiflexion, and 5?and 10?of plantarflexion using a robot-based joint testing system (RJTS), before and after subtalar arthrodesis. The RJTS enabled unconstrained A/P drawer testing at the prescribed ankle position while keeping the proximal/distal and lateral/medial forces, and varus/valgus and internal/external moments to be zero. This was achieved via a force-position hybrid control method with force and moment control, which has been shown to be more accurate than other existing force-position hybrid control methods. The target A/P force applied during the A/P drawer test was 100N in both anterior and posterior directions. The stiffness and laxity were calculated from the measured force and displacement data. The anterior and posterior stiffness of the talocrural joint were defined as the slope beyond 30% of the target A/P force, and the peak displacements quantified the laxity of the joint. Comparisons of laxity and stiffness between the intact and fusion ankle specimens were performed using Wilcoxon signed rank test (SPSS 19.0, IBM, USA) and a significance level of 0.05 was set.

Subtalar arthrodesis did not lead to significant changes in the stiffness and laxity in both anterior and posterior directions (P>0.05). The mean anterior stiffness before arthrodesis was 9.54±1.17 N/mm and was 10.35±2.40 N/mm after arthrodesis. The mean anterior displacements before and after arthrodesis were 9.68±0.94 mm and 8.97±1.42 mm, respectively.

Subtalar arthrodesis did not show significant effects on the A/P laxity and stiffness of the talocrural joint in both anterior and posterior directions. This may imply that the motion of the subtalar joints do not have significant effects on the A/P stability of the talocrural joint, which is the main joint of the ankle complex. This agrees with the anatomical roles of the subtalar joints which provide mainly the varus/valgus motions for the ankle complex. The current study provides a basis for further studies needed to evaluate the effects subtalar arthrodesis on the varus/valgus stability.