Inadequate placement of osteochondral plugs may induce abnormal stress-strain distributions in articular cartilage --finite element simulations.

The transplantation of osteochondral (cartilage-bone) plugs is an alternative approach to treat local, full thickness cartilage defects in young patients. It is technically difficult to control the amount of the press fit tolerance and the position of the osteochondral (OC) plug in the recipient hole. Inadequate placement of the OC plugs may produce abnormal stress and strain distributions within the cartilage, and thus influence the regeneration of the injured cartilage site and the maintenance of opposing, healthy cartilage surfaces. In the present study, the influence of press fit tolerance and the placement of the OC plug on the joint contact mechanics was simulated using finite element methods. The joint was assumed to be axi-symmetric with a spherical femur and tibia and a cylindrical OC plug. Our simulations showed that small misplacements of the OC plug induced abnormal tension in the articular cartilage of the opposing, healthy cartilage surface. Such tension might induce unpredictable adaptations, or possibly degenerations, in the opposing cartilage layer. The contact stress profiles in the joint were predicted to change discontinuously across the plug/recipient interface, even when the plug was perfectly placed in the recipient hole, i.e., the plug's surface was aligned with the recipient surface. For a fixed coefficient of friction and a fixed fit tolerance, the maximal sliding force was predicted to vary with the size of the plug and reached a maximum at a specific plug diameter. The present simulations should be helpful for the design of instruments for osteochondral transplantation and placement of OC plugs, for understanding articular cartilage adaptation following osteochondral repair, and for providing insight into the mechanics at the transplant/recipient interface where proper integration of the plug into the joint is most problematic.

In situ compressive stiffness, biochemical composition, and structural integrity of articular cartilage of the human knee joint.

OBJECTIVE: Reduction of compressive stiffness of articular cartilage has been reported as one of the first signs of cartilage degeneration. For the measurement of in situ compressive stiffness, a hand-held indentation probe has recently been developed and baseline data for macroscopically normal knee joint cartilage were provided. However, the histological stage of degeneration of the measured cartilage was not known. The purpose of this study was to investigate whether there is a relationship between the in situ measured compressive stiffness, the histological stage of degeneration, and the biochemical composition of articular cartilage. DESIGN: Instantaneous compressive stiffness was measured for the articular cartilage of 24 human cadaver knees. Additionally, biochemical composition (total proteoglycan and collagen content) and histological appearance (according to the Mankin score) were assessed for each measurement location. RESULTS: Despite visually normal surfaces, various histological signs of degeneration were present. A high correlation between Mankin score and cartilage stiffness was observed for the lateral patellar groove (R(2)=0.81), the medial (R(2)=0.83) and the lateral femoral condyle (R(2)=0.71), whereas a moderate correlation was found for the medial patellar groove (R(2)=0.44). No correlation was observed between biochemical composition and cartilage compressive stiffness. CONCLUSIONS: Our results are in agreement with others and show that the instantaneous compressive stiffness is primarily dependent on the integrity of the extracellular matrix, and not on the content of the major cartilage constituents. The high correlation between stiffness and Mankin score in mild osteoarthrosis suggests that the stage of cartilage degeneration can be assessed quantitatively with the hand-held indentation probe. Moderate and severe case of osteoarthrosis remains to be investigated.

Articular cartilage biomechanics: theoretical models, material properties, and biosynthetic response.

Articular cartilage has unique material properties that enable the cartilage to perform its physiological functions over a lifetime and under a wide range of loading conditions. Numerous studies have investigated the relationship between cartilage properties and composition/structure. For cartilage transplantation and regeneration, it is necessary to know how cartilage maintains its functionality and how cartilage responds to the ever-changing mechanical environment. In this review, we discuss theoretical and experimental studies on the behavior of articular cartilage to load. In the first part, the composition and structure of articular cartilage is presented. In the second part, theoretical models of the mechanical behavior of cartilage, experimental methods for the determination of cartilage properties, and material properties for normal, pathologic, and repair cartilage are summarized. In the third part, the relationship between mechanical loading of the cells and their corresponding biological responses are discussed. The goal for treating joint degeneration in the future lies in cartilage regeneration rather than prosthetic replacement. In order to achieve this goal, it has to be understood how structure and function, metabolic and biochemical properties, and biomechanical performance of articular cartilage can be restored.

In vivo knee joint loading and kinematics before and after ACL transection in an animal model.

Osteoarthritis (OA) is typically diagnosed in humans in its final stage when joint movement becomes painful. Clinical information about the onset and the mechanisms triggering the degenerative responses are virtually non-existent. However, research on animal models of experimental OA shows that joint adaptations associated with the onset of OA can be detected as early as two to four weeks following disruption of the normal joint mechanics. Transection of the anterior cruciate ligament (ACL) has been shown to cause OA-like symptoms in various animal models including the cat. However. the changes in joint loading responsible for the early tissue responses have not been quantified in vivo. Consequently, the relationship between abnormal joint loading and the onset of OA remains unknown. The purpose of this study was to quantify knee loading before and early after ACL transection in the cat. Knee mechanics were assessed by measuring patellar tendon forces, gastrocnemius forces, knee flexor and extensor EMGs, and hindlimb kinematics before and 5, 7, and 9 days following ACL transection in six experimental and two sham-operated animals. The knee mechanics were not affected by sham-surgery but the muscular forces. knee extensor EMGs, and knee range of motion were reduced following ACL transection compared to corresponding pre-intervention values. These results suggest that ACL transection causes a general unloading and changed kinematics of the knee. We speculate that the decrease in loading and the altered kinematics are responsible for the onset of biologic adaptations of the knee. Precise data about the local joint contact mechanics before and after ACL transection are now required to further relate the detailed changes in the knee mechanics to the early joint changes.

Quantification of in vivo patellofemoral contact forces before and after ACL transection.

Altered knee loading following anterior cruciate ligament (ACL) transection is believed to play an important role in initiating cartilage degeneration. Changes of in vivo joint contact forces pre- and post-ACL transection have not been quantified to date. Consequently, it is not known how knee loading changes following ACL transection, and how it contributes to cartilage degeneration. The objective of this study was to quantify in vivo patellofemoral contact forces in the cat knee prior to and up to nine days following uni-lateral ACL transection. Patellofemoral contact forces were predicted using a planar three-force model with knee extensor forces and patellofemoral geometry as input. Patellofemoral movements were expressed as functions of external knee kinematics. Kinematics and knee extensor forces were measured in both hindlimbs before and after ACL transection during unrestrained locomotion. Following ACL transection, resultant patellofemoral contact forces were decreased by approximately 30% in the ACL-deficient hindlimbs. These results suggest that decreased loading in the ACL-deficient knees may initiate the early degenerative changes observed in cartilage of ACL-transected animals. It remains to be shown, if the general unloading of the joint also results in locally decreased contact loads and altered joint kinematics. Alterations of in vivo patellofemoral loading following ACL transection have been quantified for the first time in this study. A next step will be to quantify the dynamic in vivo cartilage stresses in intact and injured knees which may help to elucidate the effects of mechanical stimuli on cartilage metabolism.

In-situ calibration of the implantable force transducer.

Recently, an implantable force transducer (IFT) has been introduced [Xu et al. (1992, J. Biomech. Engng 114, 170-177)] which can be used in tight spaces where force recordings with established transducers, such as the buckle-type transducers, are not possible because of impingement artifacts. The IFT is easily implanted in chronic animal preparations; however, calibration of the IFT in terminal experiments has produced unreliable results. The problems of IFT calibration are that minute movements of the transducer within the tendon, slight misalignments of the tendon, or slight errors in the line of pull cause dramatic changes in the IFT voltage output for a given applied calibration load. Here, we propose a method that eliminates the above calibration problems primarily because the target tendon is left in situ, the calibration loads are applied by the muscles which insert into the target tendon, and the transducer is implanted into the target tendon about two weeks prior to calibration. The theoretical and experimental approaches are demonstrated for the cat patellar tendon, but in principle can be performed with any tendon. The results are repeatable, lie within expected values, and reproduce some of the basic properties which have been observed in prior IFT testing.

Experimental evaluation of theoretical contact forces in the cat patellofemoral joint.

The purpose of this study was to evaluate the accuracy of patellofemoral contact forces predicted from a planar model of the patella by comparison with experimentally determined in situ contact forces. Patellofemoral contact pressures and areas were measured experimentally in an animal preparation with pressure sensitive film. Patellar tendon forces and lines of action used as input to the model were measured in the intact joint of the same preparation. Predicted and measured contact forces at different joint loads were compared at three different joint angles using linear regression analysis. r2-coefficients ranged from 0.94 to 0.95, and the slopes of the regression lines ranged from 1.64 to 2.11 for the three joint angles. The high r2-coefficients for all comparisons indicate that both methods were able to quantify the relative changes in the cat patellofemoral contact forces under different loading conditions accurately. However, the consistent finding of slopes greater than 1.0 indicates that the measured contact forces were systematically larger than the corresponding predicted forces. Analysis of the possible sources for the observed discrepancies between predicted and measured contact forces suggested that the directly measured patellar tendon forces were the most likely candidate causing the systematic differences. The results of this study suggest that a relatively simple model of the patellofemoral joint appears to be valid to quantify joint contact forces if appropriate patellar tendon force values can be provided as input to the model.

Appropriateness of plane pressure-sensitive film calibration for contact stress measurements in articular joints.

OBJECTIVE: To examine how accurately calibration stains from pressure-sensitive film samples compressed between perfectly flat surfaces represent contact pressures measured with the film inserted between curved joint surfaces. DESIGN: Experimental study. BACKGROUND: Contact pressure distributions and areas in joints are often quantified using pressure-sensitive film inserted between the articular surfaces. Absolute pressure values are recovered from the film through a calibration procedure. Since the calibration is typically performed in a different environment from the experimental measurements, a systematic error in the pressure magnitudes may be introduced. METHODS: Calibration stains were produced with an Instron machine using a flat and a slightly curved indenter. Calibration curves were fitted to the raw data of both indenters using a model representing the functional characteristics of the pressure-sensitive film. RESULTS: No systematic differences between the calibration curves from the flat and the curved indenter were observed. CONCLUSIONS: It appears that calibration with the film in a flat configuration is adequate for contact pressure measurements between moderately curved articular surfaces.

Influence of hip and knee joint angles on excitation of knee extensor muscles.

The purpose of this study was to test if (and how) maximal excitation of the knee extensor muscles rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) is influenced by knee and/or hip joint angles. Excitation was quantified using surface electromyography. Isometric knee extensions were performed at systematically varying knee and hip joint configurations using a strength testing machine. The results indicate that excitation of the one-joint knee extensor muscles (VL and VM) depends systematically on hip joint angles. In particular, excitation levels are higher at hip joint angles of 90 degrees (sitting) and 180 degrees (lying) compared to intermediate hip joint angles (112 degrees, 135 degrees, 157 degrees). Furthermore, it was found that excitation of all knee extensor muscles tested is higher near full knee extension (170 degrees) compared to an intermediate knee joint angle (130 degrees). Since knee extensor moments are much smaller at knee joint angles of 170 degrees compared to those at 130 degrees, it is speculated that the high excitation observed near full knee extension constitutes a neurophysiological compensation mechanism for the reduced force production ability of the muscles at this joint configuration.

Forces exerted during spinal manipulative therapy.

Spinal manipulative therapy has been widely recognized in the medical fields as a conservative treatment modality for spinal dysfunction and pain. Spinal manipulative therapy consists of an application of a thrusting force on a specific part of the spine in a well-defined direction. The magnitude of this force has been associated with positive treatment effects, such as realigning vertebral bodies, mobilizing spinal joints, relaxing back musculature through reflex pathways, and producing a respiratory burst. However, direct force measurements during spinal manipulative therapy in a clinically relevant situation have not been performed to date. The purpose of this study was to measure the forces exerted onto patients during spinal manipulative therapy on various locations of the spinal column. Force measurements were obtained using a thin, flexible pressure mat. The results indicate that peak and preload forces are considerably smaller for spinal manipulative therapy performed on the cervical spine compared to corresponding values obtained on the thoracic spine and sacroiliac joint. Furthermore, for treatments on the thoracic spine and sacroiliac joint, a significant relation was found to exist between preload and peak forces.

Forces required to cause cavitation during spinal manipulation of the thoracic spine.

The purpose of this study was to measure the forces exerted during spinal manipulative therapy of the thoracic spine simultaneously with corresponding cavitation signals. Forces were measured using a thin, flexible pressure mat which was placed on patients over the contact area between doctor and patient. Cavitation signals were measured using a skin mounted accelerometer on the spinous process of a vertebral body adjacent to the manipulated vertebral body. Mean forces of spinal manipulative therapy at the instant of cavitation were 364 N with a standard deviation of 106 N. These values are considerably larger than corresponding values reported for cavitation at metacarpophalangeal joints. The precise factors causing cavitation of the spinal joints could not be determined. Study designs which may allow identification of these factors are suggested.

Forces generated during spinal manipulative therapy of the cervical spine: a pilot study.

OBJECTIVE: To determine the forces imparted to the cervical spine using direct sampling methods during a clinical episode of spinal manipulative therapy. DESIGN: Quantitative study. SETTING: Human Performance Laboratory, University of Calgary. PARTICIPANTS: Two doctor/patient pairs. Patients were selected by the treating chiropractors from their existing patients pools. INTERVENTIONS: SMT to the cervical spine (toggle method) on three separate occasions over a 2-wk period. The clinical relevancy of the treatment was assessed via before and after measures of tissue compliance. MAIN OUTCOME MEASURE: a) Forces during manipulation: preload and peak forces. b) Duration of applied forces. RESULTS: a) Mean peak force = 117.7 N (+/- 15.6 N). b) Mean duration of force = 101.7 msec (+/- 14.7 msec). CONCLUSION: The forces obtained with direct sampling methods compare favorably to previous measurements obtained from indirect sampling techniques, yet the force duration times are smaller (faster) using the direct method.