In vivo kinematic comparison before and after mobile-bearing unicompartmental knee arthroplasty during high-flexion activities

BackgroundMany patients who undergo unicompartmental knee arthroplasty (UKA) have an expectation that their knee flexion would increase following its replacement. Additionally, the survival rate of mobile-bearing UKA (MB-UKA) is high. However, the effect on the patient's kinematics remains unknown. This study aimed to clarify the kinematic effect of MB-UKA knees during high-flexion activities by comparing the in vivo kinematics before and after surgery.MethodsA squatting motion was performed under fluoroscopic surveillance in the sagittal plane before and after MB-UKA. To estimate the spatial position and orientation of the knee, a two-dimensional/three-dimensional registration technique was used. The femoral rotation and varus–valgus angle relative to the tibia and anteroposterior (AP) translation of the medial and lateral side of the femur on the plane perpendicular to the tibial mechanical axis in each flexion angle were evaluated.ResultsRegarding the varus–valgus angle, the preoperative knees indicated a significant varus alignment compared with the postoperative knees from 10° to 60° of flexion. There were no significant differences in the femoral rotation angle, AP translation, and kinematic pathway before and after MB-UKA in the mid-flexion of the range of motion.ConclusionThere were differences between the varus–valgus knee kinematics before and after MB-UKA, from 10 to 60° of flexion, but no difference from midrange of flexion to deep flexion. In addition, the rotational knee kinematics before and after MB-UKA was not significantly different.     

The effect of screw insertion angle and thread type on the pullout strength of bone screws in normal and osteoporotic cancellous bone models

Screw fixation can be extremely difficult to achieve in osteoporotic (OP) bone because of its low strength. This study determined how pullout strength is affected by placing different bone screws at varying angles in normal and OP bone models. Pullout tests of screws placed axially, and at angles to the pullout axis (ranging from 10° to 40°), were performed in 0.09 g cm^?3, 0.16 g cm^?3 and 0.32 g cm^?3 polyurethane (PU) foam. Two different titanium alloy bone screws were used to test for any effect of thread type (i.e. cancellous or cortical) on the screw pullout strength. The cancellous screw required a significantly higher pullout force than the cortical screw (p < 0.05). For both screws, pullout strength significantly increased with increasing PU foam density (p < 0.05). For screws placed axially, and sometimes at 10°, the observed mechanism of failure was stripping of the internal screw threads generated within the PU foam by screw insertion. For screws inserted at 10°, 20°, 30° and 40°, the resistance to pullout force was observed to be by compression of the PU foam material above the angled screw; clinically, this suggests that compressed OP bone is stronger than unloaded OP bone.     

Lateral patellar maltracking due to trochlear dysplasia: A computational study

BackgroundThe study focuses on the influence of trochlear dysplasia on patellar tracking related to patellar instability.MethodsKnee extension against gravity and dual-limb squatting were simulated with seven models representing knees being treated for recurrent instability. Trochlear depth was altered to represent lateral trochlear inclination (LTI) values of 6°, 12° and 24°. Repeated measures analyses compared patellar lateral shift (bisect offset index) across different LTI values. Peak bisect offset index during extension and squatting was correlated with patella alta (Caton–Deschamps index) and maximum lateral position of the tibial tuberosity.ResultsBisect offset index varied significantly (p < 0.05) between different LTI values at multiple flexion angles throughout simulated knee extension and squatting. Average bisect offset values were 1.02, 0.95, and 0.86 for LTI = 6°, 12°, and 24°, respectively, at 0° of flexion for knee extension. The strongest correlation occurred between peak bisect offset index and lateral position of the tibial tuberosity for knee squatting with LTI = 6° (r² = 0.81, p = 0.006). The strength of the correlation decreased as LTI increased. Caton–Deschamps was only significantly correlated with patellar tracking for LTI = 24° during knee squatting.ConclusionsA shallow trochlear groove increases lateral patellar maltracking. A lateral tibial tuberosity in combination with trochlear dysplasia increases lateral patellar tracking and the risk of patellar instability. Patella alta has relatively little influence on patellar tracking in combination with trochlear dysplasia due to the limited articular constraint provided by the trochlear groove.     

Prophylactic vertebroplasty can decrease the fracture risk of adjacent vertebrae: An in vitro cadaveric study

Adjacent level vertebral fractures are common in patients with osteoporotic wedge fractures, but can theoretically be prevented with prophylactic vertebroplasty. Previous tests on prophylactic vertebroplasties have been performed under axial loading, while in vivo changes in spinal alignment likely cause off-axis loads. In this study we determined whether prophylactic vertebroplasty can also reduce the fracture risk under off-axis loads.In a previous study, we tested vertebral bodies that were loaded axially or 20° off-axis representing vertebrae in an unfractured spine or vertebrae adjacent to a wedge fracture, respectively. In the current study, vertebral failure load and stiffness of our previously tested vertebral bodies were compared to those of a new group of vertebral bodies that were filled with bone cement and then loaded 20° off-axis. These vertebral bodies represented adjacent-level vertebrae with prophylactic bone cement filling.Prophylactic augmentation resulted in failure loads that were comparable to those of the 0° group, and 32% greater than the failure loads of the 20° group. The stiffness of the prophylacticly augmented vertebrae was 21% lower than that of the 0° group, but 27% higher than that of the 20° group. We conclude that prophylactic augmentation can decrease the fracture risk in a malaligned, osteoporotic vertebra. Whether this is enough to actually prevent additional vertebral fractures in vivo remains subject of further study.     

Tibial displacement and rotation during seated knee extension and wall squatting: A comparative study of tibiofemoral kinematics between chronic unilateral anterior cruciate ligament deficient and healthy knees

BackgroundFollowing anterior cruciate ligament (ACL) rupture, the knee becomes unstable with alterations in joint kinematics including anterior tibial displacement (ATD), and internal tibial rotation. Therapeutic exercises that promote faulty kinematics should be discouraged, especially early post-reconstruction, to avoid graft stretching and possibly longer-term osteoarthritis. Our study aimed to compare ATD and tibial rotation during two commonly prescribed exercises, namely: open kinetic chain (OKC) seated extension and closed kinetic chain (CKC) single leg wall squatting in ACL-deficient and healthy knees.MethodsEight ACL-deficient patients and eight healthy subjects matched for age, gender and sports history were assessed using Qualisys 3D-Motion Analysis System to track 17 infrared markers while performing a seated knee extension with 3 kg weight and a unilateral wall squat. We developed a model to measure joint kinematics through 70° of knee flexion and extension. ANOVA and paired t-tests compared relative ATD and tibial rotation between exercises and groups at 10° increments of flexion and extension.ResultsWe found increased ATD in the wall squat compared to the seated extension (p = 0.049). There was no difference in ATD between the healthy and ACL-deficient knees but overall the tibia was significantly more internally rotated (p = 0.003) in ACL-deficient knees, irrespective of the exercise, possibly interfering with the screw-home mechanism.ConclusionsCKC exercises, in particular wall squats, are not necessarily safer for patients with ACL-deficiency and possibly ACL-reconstruction; although generalization should only be made with appropriate caution. Clinicians require a detailed knowledge of the effect of exercise on knee joint kinematics.     

In vivo bone strain and finite element modeling of the mandible of Alligator mississippiensis

Forces experienced during feeding are thought to strongly influence the morphology of the vertebrate mandible; in vivo strain data are the most direct evidence for deformation of the mandible induced by these loading regimes. Although many studies have documented bone strains in the mammalian mandible, no information is available on strain magnitudes, orientations or patterns in the sauropsid lower jaw during feeding. Furthermore, strain gage experiments record the mechanical response of bone at a few locations, not across the entire mandible. In this paper, we present bone strain data recorded at various sites on the lower jaw of Alligator mississippiensis during in vivo feeding experiments. These data are used to understand how changes in loading regime associated with changes in bite location are related to changes in strain regime on the working and balancing sides of the mandible. Our results suggest that the working side mandible is bent dorsoventrally and twisted about its long‐axis during biting, and the balancing side experiences primarily dorsoventral bending. Strain orientations are more variable on the working side than on the balancing side with changes in bite point and between experiments; the balancing side exhibits higher strain magnitudes. In the second part of this paper, we use principal strain orientations and magnitudes recorded in vivo to evaluate a finite element model of the alligator mandible. Our comparison demonstrates that strain orientations and mandibular deformation predicted by the model closely match in vivo results; however, absolute strain magnitudes are lower in the finite element model.     

A Novel Testing Platform for Assessing Knee Joint Mechanics: A Parallel Robotic System Combined with an Instrumented Spatial Linkage

Assessing joint function following trauma and its inter-relation with degenerative changes requires an understanding of the normal state of structural loading in the joint. Very few studies have attempted to reproduce joint specific in vivo motions in vitro to quantify the actual loads carried by different tissues within the knee joint. The most significant challenge in this area is the very high sensitivity of the loads in joint structures to motion reproduction accuracy. A novel testing platform for assessing knee joint mechanics is described, comprised of a highly accurate (0.3 ± 0.1 mm, 0.3 ± 0.1°) six-degree-of-freedom (6-DOF) instrumented spatial linkage (ISL) for in vivo joint kinematic assessments and a unique 6-DOF parallel robotic manipulator. A position feedback system (ISL and position controller) is used for accurate reproduction of in vivo joint motions and estimation of “in situ” joint/tissue loads. The parallel robotic manipulator provides excellent stiffness and repeatability in reproducing physiological motions in 6-DOF, compared to the commonly used serial robots. The position feedback system provides real-time feedback data to the robot to reproduce in vivo motions and significantly enhances motion reproduction accuracy by adjusting for robot end-effector movements. Using this combined robot-ISL system, in vivo motions can be reproduced in vitro with very high accuracy (0.1 mm, 0.1°). Our results indicate that this level of accuracy is essential for meaningful estimation of tissue loads during gait. Using this novel testing platform, we have determined the normal load-carrying characteristics of different tissues within the ovine knee joint. The application of this testing system will continue to increase our understanding of normal and pathological joint states.     

The quality of bone surfaces may govern the use of model based fluoroscopy in the determination of joint laxity

The assessment of knee joint laxity is clinically important but its quantification remains elusive. Calibrated, low dosage fluoroscopy, combined with registered surfaces and controlled external loading may offer possible solutions for quantifying relative tibio-femoral motion without soft tissue artefact, even in native joints. The aim of this study was to determine the accuracy of registration using CT and MRI derived 3D bone models, as well as metallic implants, to 2D single-plane fluoroscopic datasets, to assess their suitability for examining knee joint laxity.Four cadaveric knees and one knee implant were positioned using a micromanipulator. After fluoroscopy, the accuracy of registering each surface to the 2D fluoroscopic images was determined by comparison against known translations from the micromanipulator measurements. Dynamic measurements were also performed to assess the relative tibio-femoral error. For CT and MRI derived 3D femur and tibia models during static testing, the in-plane error was 0.4 mm and 0.9 mm, and out-of-plane error 2.6 mm and 9.3 mm respectively. For metallic implants, the in-plane error was 0.2 mm and out-of-plane error 1.5 mm. The relative tibio-femoral error during dynamic measurements was 0.9 mm, 1.2 mm and 0.7 mm in-plane, and 3.9 mm, 10.4 mm and 2.5 mm out-of-plane for CT and MRI based models and metallic implants respectively. The rotational errors ranged from 0.5° to 1.9° for CT, 0.5–4.3° for MRI and 0.1–0.8° for metallic implants.The results of this study indicate that single-plane fluoroscopic analysis can provide accurate information in the investigation of knee joint laxity, but should be limited to static or quasi-static evaluations when assessing native bones, where possible. With this knowledge of registration accuracy, targeted approaches for the determination of tibio-femoral laxity could now determine objective in vivo measures for the identification of ligament reconstruction candidates as well as improve our understanding of the consequences of knee joint instability in TKA.     

Intraoperative kinematics of bicruciate-stabilized total knee arthroplasty during high-flexion motion of the knee

BackgroundIt is unknown whether intraoperative kinematics of bicruciate-stabilized total knee arthroplasty (BCS-TKA) are different for different activities. It has also not been established whether intraoperative high-flexion motions correlate with postoperative patient-reported outcome measures (PROMs). We aimed to clarify the intraoperative kinematics of BCS-TKA during high-flexion activities and describe the relationship between intraoperative and postoperative patient-reported outcomes.MethodsWe examined 33 knees from 31 patients who underwent BCS-TKA and measured intraoperative knee kinematics, passive knee flexion, and cross-legged flexion using a navigation system. We also calculated knee flexion, varus-valgus, and rotation angles. As a secondary evaluation, we divided the patients into two clusters based on the PROMs and compared the kinematics between them.ResultsThe valgus moved by 1.3 ± 1.3° beyond 90° knee flexion during passive flexion. In contrast, during cross-legged flexion, the varus moved by 4.6 ± 5.1° beyond 30° flexion. This indicated significantly increased varus alignment in the cross-legged flexion as compared with passive flexion. Beyond 60° of flexion, the femur displayed 8.8 ± 4.8° of external rotation relative to the tibia. In cross-legged flexion, the femur displayed 9.2 ± 6.5° of external rotation relative to the tibia beyond 45° of flexion. At 90° of flexion, the cross-legged knees rotated more externally. There were no significant postoperative differences between the high- and low-score clusters.ConclusionThe intraoperative knee kinematics after BCS-TKA during high-flexion motions differed depending on the performance of an individual. This will be useful for physicians who might recommend BCS-TKA to new patients.     

In vivo measurements of patellar tracking and finite helical axis using a static magnetic resonance based methodology

Patellofemoral (PF) maltracking is a critical factor predisposing to PF pain syndrome. Many novel techniques of measuring patellar tracking remain research tools. This study aimed to develop a method to measure the in vivo patellar tracking and finite helical axis (FHA) by using a static magnetic resonance (MR) based methodology. The geometrical models of PF joint at 0°, 45°, 60°, 90°, and 120° of knee flexion were developed from MR images. The approximate patellar tracking was derived from the discrete PF models with a spline interpolation algorithm. The patellar tracking was validated with the previous in vitro and in vivo experiments. The patellar FHA throughout knee flexion was calculated. In the present case, the FHA drew an “L-shaped” curve in the sagittal section. This methodology could advance the examination of PF kinematics in clinics, and may also provide preliminary knowledge on patellar FHA study.     

Investigating the Longitudinal Effect of Ovariectomy on Bone Properties Using a Novel Spatiotemporal Approach

Osteoporosis is the most common bone disease. However, the mechanism of osteoporosis-induced alterations in bone is still unclear. The aim of this study was to investigate the effects of osteoporosis on the structural, densitometric and mechanical properties of the whole tibia using in vivo μCT imaging, spatiotemporal analysis and finite element modeling. Twelve C57Bl/6 female mice were adopted. At 14 weeks of age, half of the mice were ovariectomized (OVX), and the other half were SHAM-operated. The whole right tibia was scanned using an in vivo μCT imaging system at 14, 16, 17, 18, 19, 20, 21 and 22 weeks. The image datasets were registered in order to precisely quantify the bone properties. The results showed that OVX led to a significant increase in the endosteal area across the whole tibia 4 weeks after OVX intervention but did not have a significant influence on the periosteal area. Additionally, the bone volume and mineral content significantly decreased only in the proximal regions, but these decreases did not have a significant influence on the stiffness and failure load of the tibia. This study demonstrated the application of a novel spatiotemporal approach in the comprehensive analysis of bone adaptations in the spatiotemporal space.     

In vitro and in vivo studies on the degradation of high-purity Mg (99.99wt.%) screw with femoral intracondylar fractured rabbit model

High-purity magnesium (HP Mg) takes advantage in no alloying toxic elements and slower degradation rate in lack of second phases and micro-galvanic corrosion. In this study, as rolled HP Mg was fabricated into screws and went through in vitro immersion tests, cytotoxicity test and bioactive analysis. The HP Mg screws performed uniform corrosion behavior in vitro, and its extraction promoted cell viability, bone alkaline phosphatase (ALP) activity, and mRNA expression of osteogenic differentiation related gene, i.e. ALP, osteopontin (OPN) and RUNX2 of human bone marrow mesenchymal stem cells (hBMSCs).Then HP Mg screws were implanted in vivo as load-bearing implant to fix bone fracture and subsequently gross observation, range of motion (ROM), X-ray scanning, qualitative micro-computed tomography (μCT) analysis, histological analysis, bending-force test and SEM morphology of retrieved screws were performed respectively at 4, 8, 16 and 24 weeks. As a result, the retrieved HP Mg screws in fixation of rabbit femoral intracondylar fracture showed uniform degradation morphology and enough bending force. However, part of PLLA screws was broken in bolt, although its screw thread was still intact. Good osseointegration was revealed surrounding HP Mg screws and increased bone volume and bone mineral density were detected at fracture gap, indicating the rigid fixation and enhanced fracture healing process provided by HP Mg screws. Consequently, the HP Mg showed great potential as internal fixation devices in intra-articular fracture operation.     

Towards understanding knee joint laxity: Errors in non-invasive assessment of joint rotation can be corrected

The in vivo quantification of rotational laxity of the knee joint is of importance for monitoring changes in joint stability or the outcome of therapies. While invasive assessments have been used to study rotational laxity, non-invasive methods are attractive particularly for assessing young cohorts. This study aimed to determine the conditions under which tibio-femoral rotational laxity can be assessed reliably and accurately in a non-invasive manner.The reliability and error of non-invasive examinations of rotational joint laxity were determined by comparing the artefact associated with surface mounted markers against simultaneous measurements using fluoroscopy in five knees including healthy and ACL deficient joints. The knees were examined at 0°, 30°, 60° and 90° flexion using a device that allows manual axial rotation of the joint. With a mean RMS error of 9.6°, the largest inaccuracy using non-invasive assessment was present at 0° knee flexion, whereas at 90° knee flexion, a smaller RMS error of 5.7° was found. A Bland and Altman assessment indicated that a proportional bias exists between the non-invasive and fluoroscopic approaches, with limits of agreement that exceeded 20°. Correction using average linear regression functions resulted in a reduction of the RMS error to below 1° and limits of agreement to less than ±1° across all knees and flexion angles.Given the excellent reliability and the fact that a correction of the surface mounted marker based rotation values can be achieved, non-invasive evaluation of tibio-femoral rotation could offer opportunities for simplified devices for use in clinical settings in cases where invasive assessments are not justified. Although surface mounted marker based measurements tend to overestimate joint rotation, and therefore joint laxity, our results indicate that it is possible to correct for this error.     

The effect of loading in mechanical response predictions of bone lengthening

Intramedullary (IM) distractor nails have become a viable alternative in bone-distraction operations. Upon stabilization of the fractured/dissected limb via the nail, the resulting construct accommodates the load bearing function of the otherwise healthy limb. In establishing design performance targets for these devices, in vitro test conditions are widely accepted leaving the in vivo conditions aside. However, in vivo device failures due to distraction forces necessitate novel modeling considerations. It is especially important to simulate the loads in limb distraction, as this brings the bone-implant construct to a totally different regime than the hip-joint contact force (Point-Force Model, PFM). In this work we used a simplified approach to incorporate ligament stretching due to limb distraction via self-equilibrating spring elements in a finite-element setting (spring-force model, SFM). We compared the effect of loading type on load transmission paths through the locking pins, for these two distinct loading modes, namely, SFM and PFM. The two modes illustrate entirely different load transfer regimes around the bone/nail interface region. In order to avoid high contact stresses between the nail and the bone segments, it is advisable to keep the osteotomy away from the mid-range between the pin connections. It was also seen for both loading modes that including an additional pin at a load transfer location does not significantly alter the load carried by a single pin (the additional pin rather acts as a geometric stabilizer).     

In vivo kinematic determination of total knee arthroplasty from squatting to standing

This study analyzed three dimensional (3D) in vivo kinematic data from the squatting to standing position for 18 Japanese subjects (18 knees) implanted with either Legacy^® Posterior Stabilized (LPS) Flex Fixed Bearing TKA or LPS Flex Mobile Bearing TKA. Under weight-bearing conditions, for all patients, the average roll-forward motions for the medial and lateral condyles were 4.0 ± 3.6 mm and 6.3 ± 3.4 mm, and the average external axial rotation was 3.1° ± 4.1°. For both groups, the weight-bearing range-of-motion (ROM) (110.7° ± 12.7°) was less than pre (127.2 ± 15.5°) and post (135.6 ± 5.4°) operative non-weight bearing ROM. As hypothesized, the incidence, average and maximum liftoff for the squatting to standing activity were much less than those of deep knee bend (DKB), and condylar motions and kinematics were opposite that of the DKB. There was little statistical difference of their kinematic patterns during this activity between the LPS fixed and mobile TKA implants.     

Patient-tailored plate for bone fixation and accurate 3D positioning in corrective osteotomy

A bone fracture may lead to malunion of bone segments, which gives discomfort to the patient and may lead to chronic pain, reduced function and finally to early osteoarthritis. Corrective osteotomy is a treatment option to realign the bone segments. In this procedure, the surgeon tries to improve alignment by cutting the bone at, or near, the fracture location and fixates the bone segments in an improved position, using a plate and screws. Three-dimensional positioning is very complex and difficult to plan, perform and evaluate using standard 2D fluoroscopy imaging. This study introduces a new technique that uses preoperative 3D imaging to plan positioning and design a patient-tailored fixation plate that only fits in one way and realigns the bone segments as planned. The method is evaluated using artificial bones and renders realignment highly accurate and very reproducible (d _err < 1.2 ± 0.8 mm and φ _err < 1.8° ± 2.1°). Application of a patient-tailored plate is expected to be of great value for future corrective osteotomy surgeries.     

Kinematic description of soft tissue artifacts: quantifying rigid versus deformation components and their relation with bone motion

This paper proposes a kinematic approach for describing soft tissue artifacts (STA) in human movement analysis. Artifacts are represented as the field of relative displacements of markers with respect to the bone. This field has two components: deformation component (symmetric field) and rigid motion (skew-symmetric field). Only the skew-symmetric component propagates as an error to the joint variables, whereas the deformation component is filtered in the kinematic analysis process. Finally, a simple technique is proposed for analyzing the sources of variability to determine which part of the artifact may be modeled as an effect of the motion, and which part is due to other sources. This method has been applied to the analysis of the shank movement induced by vertical vibration in 10 subjects. The results show that the cluster deformation is very small with respect to the rigid component. Moreover, both components show a strong relationship with the movement of the tibia. These results suggest that artifacts can be modeled effectively as a systematic relative rigid movement of the marker cluster with respect to the underlying bone. This may be useful for assessing the potential effectiveness of the usual strategies for compensating for STA.     

Development of a subject-specific model to predict the forces in the knee ligaments at high flexion angles

Recent clinical evidence has suggested that tasks performed in kneeling or squatting postures place the knee at a higher risk for injury because loads across the knee might overload the ligaments. The objective of this study was to develop a subject-specific model of the knee that is kinematically driven to predict the forces in the major ligaments at high flexion angles. The geometry of the femur, tibia, and fibula and the load-elongation curves representing the structural properties of the ACL, PCL, LCL, and MCL served as inputs to the model, which represented each ligament as a nonlinear elastic spring. To drive the model, kinematic data was obtained while loads were applied to the same cadaveric knee at four flexion angles. The force in each ligament during the recorded kinematic data allowed an optimization procedure to determine the location of the ligament attachment sites on each bone and their reference lengths. The optimization procedure could successfully minimize the differences between the experimental and predicted forces only when the kinematics at 90°, 120°, and 140° of flexion were utilized. This finding suggests that the ligaments at the knee function differently at high-flexion angles compared to low flexion angles and separate models must be used to examine each range of motion. In the future, the novel experimental and computational methodology will be used to construct additional models and additional knee kinematics will be input to help elucidate mechanisms of injury during tasks performed in kneeling or squatting postures.     

Net change in periosteal strain during stance shift loading after surgery correlates to rapid de novo bone generation in critically sized defects

In an ovine femur model, proliferative woven bone fills critically sized defects enveloped by periosteum within 2 weeks of treatment with the one-stage bone-transport surgery. We hypothesize that mechanical loading modulates this process. Using high-definition optical strain measurements we determined prevailing periosteal strains for normal and surgically treated ovine femora subjected ex vivo to compressive loads simulating in vivo stance shifting (n = 3 per group, normal vs. treated). We determined spatial distribution of calcein green, a label for bone apposition in first the 2 weeks after surgery, in 15°, 30°, and 45° sectors of histological cross sections through the middle of the defect zone (n = 6 bones, three to four sections per bone). Finally, we correlated early bone formation to either the maximal periosteal strain or the net change in maximal periosteal strain. We found that treatment with the one-stage bone-transport surgery profoundly changes the mechanical environment of cells within the periosteum during stance shift loading. The pattern of early bone formation is repeatable within and between animals and relates significantly to the actual strain magnitude prevailing in the periosteum during stance shift loading. Interestingly, early bone apposition after the surgery correlates well to the maximal net change in strain (above circa 2000–3000 με, in tension or compression) rather than strain magnitude per se, providing further evidence that changes in cell shape may drive mechanoadaptation by progenitor cells. These important insights regarding mechanobiological factors that enhance rapid bone generation in critically sized defects can be translated to the tissue and organ scale, providing a basis for the development of best practices for clinical implementation and the definition of movement protocols to enhance the regenerative effect.     

Hamstring antagonist torque generated in vivo following ACL rupture and ACL reconstruction

Hamstring motor behaviour and resultant antagonist torque during knee extension has been quantified in uninjured individuals however, the effect of ACL rupture and ACL reconstruction (ACLR) on the morphology of hamstring antagonist torque generated in vivo is unknown. The purpose of this cross-sectional study was to quantify the hamstring antagonist torque generated in vivo during isokinetic knee extension in ACLD and ACLR patients relative to uninjured control subjects. Ten male ACL deficient (ACLD) subjects (18–35 years), 14 matched males who had undergone ACLR using the bone–patellar tendon–bone graft and 22 matched male control subjects participated. We used a mathematical model to estimate the opposing torque generated by the hamstrings during isokinetic knee extension in 10° intervals from 80° to 10° knee flexion. Control group hamstring antagonist torque was significantly lower at 80–70° knee flexion compared with that of the ACLD (% Diff = 40.2; p = 0.019) and ACLR (% Diff = 34.8; p = 0.036) groups. For all subject groups, hamstring antagonist torque demonstrated a descending–ascending curve; decreasing significantly from 80–70° to 50–40° knee flexion (% Diff = 40.8 to 63.3; p = < 0.001 to 0.009) but then increasing significantly from 50–40° to 20–10° knee flexion (% Diff = 37.6 to 59.0; p = < 0.001 to 0.012). ACL status and therefore, the ACL-hamstring reflex has little effect on the magnitude of hamstring antagonist torque generated during quadriceps-induced knee joint loading. Capsular afferents are thought to dictate the hamstring torque profile which decreased then increased during knee extension to maintain dynamic joint stability.