Tibiofemoral Forces Research Articles

Quantification of Tibiofemoral Shear and Compressive Loads Using an MRI-Based EMG-Driven Knee Model

The purpose of this study is to describe an MRI-based EMG-driven knee model to quantify tibiofemoral compressive and shear forces. Twelve healthy females participated. Subjects underwent 2 phases of data collection: (1) MRI assessment of the lower extremity to quantify muscle volumes and patella tendon orientation and (2) biomechanical evaluation of a drop-jump task. A subject-specific EMG-driven knee model that incorporated lower extremity kinematics, EMG, and muscle volumes and patella tendon orientation estimated from MRI was developed to quantify tibiofemoral shear and compressive forces. A resultant anterior tibial shear force generated from the ground reaction force (GRF) and muscle forces was observed during the first 30% of the stance phase of the drop-jump task. All of the muscle forces and GRF resulted in tibiofemoral compression, with the quadriceps force being the primary contributor. Acquiring subject-specific muscle volumes and patella tendon orientation for use in an EMG-driven knee model may be useful to quantify tibiofemoral forces in persons with altered patella position or muscle atrophy following knee injury or pathology.

Patellofemoral and tibiofemoral forces in cyclists and triathletes: effects of saddle height

High compressive load applied to the patellofemoral joint at great knee flexion angle (e.g. >60Âo of flexion), as usually observed in cycling may provide a link between body position on the bicycle and knee joint forces. The aim of the present study was to compare knee joint forces in different saddle heights. Right pedal force and lower limb kinematics were collected for 24 competitive cyclists and triathletes at self-selected preferred, high (-10Âo of knee flexion angle from preferred), low (+10Âo of knee flexion angle from preferred) and optimal (25Âo of knee flexion angle) saddle heights in submaximal pedalling cadence and workload trials (3.45 ± 0.6 W/kg, 90 ± 2 rpm and 163 ± 33 J). Patellofemoral compressive and tibiofemoral anterior-posterior and compressive force were computed by inverse dynamics and compared for different saddle heights via effect sizes. Patellofemoral compressive force (5-13%) and tibiofemoral compressive force (1-7%) were not substantially affected by changes in saddle height. Tibiofemoral anterior shear force decreased at low saddle heights (4-6% of the preferred height) compared to optimal (35%) and high saddle heights (53%). Greater knee flexion angles were observed for lower saddle heights (8-34%). Knee flexion angle was significantly affected by changes in saddle height, which may indicate that using joint kinematics to assess saddle height effects may be useful to anticipate overload in knee joint.

Gender differences in tibio-femoral kinematics and quadriceps muscle force during weight-bearing knee flexion in vitro

Females have a higher risk in terms of anterior cruciate ligament injuries during sports than males. Reasons for this fact may be different anatomy and muscle recruitment patterns leading to less protection for the cruciate- and collateral-ligaments. This in vitro study aims to evaluate gender differences in knee joint kinematics and muscle force during weight-bearing knee flexions. Thirty-four human knee specimens (17 females/17 males) were mounted on a dynamic knee simulator. Weight-bearing single-leg knee flexions were performed with different amounts of simulated body weight (BW). Gender-specific kinematics was measured with an ultrasonic motion capture system and different loading conditions were examined. Knee joint kinematics did not show significant differences regarding anteroposterior and medial-lateral movement as well as tibial varus-valgus and internal-external rotation. This applied to all simulated amounts of BW. Simulating 100 N BW in contrast to AF50 led to a significant higher quadriceps overall force in female knees from 45° to 85° of flexion in contrast to BW 50 N. In these female specimens, the quadriceps overall force was about 20 % higher than in male knees being constant in higher flexion angles. It is indicated by our results that in a squatting movement females compared with males produce higher muscle forces, suggesting an increased demand for muscular stabilization, whereas tibio-femoral kinematics was similar for both genders.

Tibiofemoral joint reaction force during the stance phase of backward- and forward-walking at variable speeds.

Background: The knee is the joint that often suffers from sport injury. Adequate post-injury rehabilitation helps the patients come back to the game earlier, and prevents re-injury. As part of the program, backward-walking is sometimes used, but information available for the knee-joint reaction force is limited. Objective: Determine tibiofemoral joint reaction force (TFJRF) during the stance phase of backward- and forwardwalking at variable speeds. Methods: Fifty-four healthy Thai males (age 20 to 39 year old, body mass index Results: Backward-walking produced higher peak TFJRF during the stance phase than that of forward-walking in every speed. The subjects had higher HR in every speed during backward-walking, but the average TFJRF was lower in all test speeds except 0.8 m/second. Conclusion: Peak TFJRF and HR during backward-walking were higher than those during forward-walking in every speed, but backward-walking showed a trend to lower the averaged TFJRF compared with forwardwalking. In clinical practice, lower speed of backward-walking may be appropriate to prescribe as an exercise for those with tibiofemoral joint problems. Keywords: Backward walking, inverse dynamics, joint reaction force, split-belt treadmill, stance phase

Compressive tibiofemoral force during crouch gait

Crouch gait, a common walking pattern in individuals with cerebral palsy, is characterized by excessive flexion of the hip and knee. Many subjects with crouch gait experience knee pain, perhaps because of elevated muscle forces and joint loading. The goal of this study was to examine how muscle forces and compressive tibiofemoral force change with the increasing knee flexion associated with crouch gait. Muscle forces and tibiofemoral force were estimated for three unimpaired children and nine children with cerebral palsy who walked with varying degrees of knee flexion. We scaled a generic musculoskeletal model to each subject and used the model to estimate muscle forces and compressive tibiofemoral forces during walking. Mild crouch gait (minimum knee flexion 20–35°) produced a peak compressive tibiofemoral force similar to unimpaired walking; however, severe crouch gait (minimum knee flexion>50°) increased the peak force to greater than 6 times body-weight, more than double the load experienced during unimpaired gait. This increase in compressive tibiofemoral force was primarily due to increases in quadriceps force during crouch gait, which increased quadratically with average stance phase knee flexion (i.e., crouch severity). Increased quadriceps force contributes to larger tibiofemoral and patellofemoral loading which may contribute to knee pain in individuals with crouch gait.

The Use of Piezoceramics As Electrical Energy Harvesters Within Instrumented Knee Implant During Walking

The goal of our paper is to quantify the electrical energy that can be harvested within a new generation of instrumented knee implant during normal walking. This generation of knee implant is proposed to assess the in vivo anteroposterior and mediolateral distributions of tibiofemoral force on the tibial baseplate without the need to be powered from an external source of energy. The proposed self-powered diagnostic knee implant can provide the clinicians with useful information on the sagittal and coronal instabilities of the prosthetic knee throughout its lifespan. Four piezoelectric elements were embedded within the anteromedial, posteromedial, anterolateral, and posterolateral compartments of the tibial baseplate. These elements can simultaneously be used to sense the force distribution and generate the electric power needed to supply the acquisition, processing, and transmission system located in the stem of the implant. In order to study the power generation issue, OrCAD/PSpice and MATLAB/Simulink models of the piezoelectric element have been developed to quantify the electrical energy harvested under operating conditions close to those encountered in vivo during normal walking. Furthermore, an experimental prototype of the self-powered diagnostic knee implant has been designed, developed, and tested in our laboratory (LaTIM, INSERM U650, Brest, France) in order to validate the modeling results.

Patellofemoral joint contact forces during activities with high knee flexion

The patellofemoral (PF) joint plays an essential role in knee function, but little is known about the in vivo loading conditions at the joint. We hypothesized that the forces at the PF joint exceed the tibiofemoral (TF) forces during activities with high knee flexion. Motion analysis was performed in two patients with telemetric knee implants during walking, stair climbing, sit-to-stand, and squat. TF and PF forces were calculated using a musculoskeletal model, which was validated against the simultaneously measured in vivo TF forces, with mean errors of 10% and 21% for the two subjects. The in vivo peak TF forces of 2.9-3.4 bodyweight (BW) varied little across activities, while the peak PF forces showed significant variability, ranging from less than 1 BW during walking to more than 3 BW during high flexion activities, exceeding the TF forces. Together with previous in vivo measurements at the hip and knee, the PF forces determined here provide evidence that peak forces across these joints reach values of around 3 BW during high flexion activities, also suggesting that the in vivo loading conditions at the knee can only be fully understood if the forces at the TF and the PF joints are considered together.

Effects of Bicycle Saddle Height on Knee Injury Risk and Cycling Performance

Incorrect bicycle configuration may predispose athletes to injury and reduce their cycling performance. There is disagreement within scientific and coaching communities regarding optimal configuration of bicycles for athletes. This review summarizes literature on methods for determining bicycle saddle height and the effects of bicycle saddle height on measures of cycling performance and lower limb injury risk. Peer-reviewed journals, books, theses and conference proceedings published since 1960 were searched using MEDLINE, Scopus, ISI Web of Knowledge, EBSCO and Google Scholar databases, resulting in 62 references being reviewed. Keywords searched included 'body positioning', 'saddle', 'posture, 'cycling' and 'injury'. The review revealed that methods for determining optimal saddle height are varied and not well established, and have been based on relationships between saddle height and lower limb length (Hamley and Thomas, trochanteric length, length from ischial tuberosity to floor, LeMond, heel methods) or a reference range of knee joint flexion. There is limited information on the effects of saddle height on lower limb injury risk (lower limb kinematics, knee joint forces and moments and muscle mechanics), but more information on the effects of saddle height on cycling performance (performance time, energy expenditure/oxygen uptake, power output, pedal force application). Increasing saddle height can cause increased shortening of the vastii muscle group, but no change in hamstring length. Length and velocity of contraction in the soleus seems to be more affected by saddle height than that in the gastrocnemius. The majority of evidence suggested that a 5% change in saddle height affected knee joint kinematics by 35% and moments by 16%. Patellofemoral compressive force seems to be inversely related to saddle height but the effects on tibiofemoral forces are uncertain. Changes of less than 4% in trochanteric length do not seem to affect injury risk or performance. The main limitations from the reported studies are that different methods have been employed for determining saddle height, small sample sizes have been used, cyclists with low levels of expertise have mostly been evaluated and different outcome variables have been measured. Given that the occurrence of overuse knee joint pain is 50% in cyclists, future studies may focus on how saddle height can be optimized to improve cycling performance and reduce knee joint forces to reduce lower limb injury risk. On the basis of the conflicting evidence on the effects of saddle height changes on performance and lower limb injury risk in cycling, we suggest the saddle height may be set using the knee flexion angle method (25-30°) to reduce the risk of knee injuries and to minimize oxygen uptake.

The 2011 ABJS Nicolas Andry Award: ‘Lab’-in-a-Knee: In Vivo Knee Forces, Kinematics, and Contact Analysis

Tibiofemoral forces are important in the design and clinical outcomes of TKA. We developed a tibial tray with force transducers and a telemetry system to directly measure tibiofemoral compressive forces in vivo. Knee forces and kinematics traditionally have been measured under laboratory conditions. Although this approach is useful for quantitative measurements and experimental studies, the extrapolation of results to clinical conditions may not always be valid. We therefore developed wearable monitoring equipment and computer algorithms for classifying and identifying unsupervised activities outside the laboratory. Tibial forces were measured for activities of daily living, athletic and recreational activities, and with orthotics and braces, during 4 years postoperatively. Additional measurements included video motion analysis, EMG, fluoroscopic kinematic analysis, and ground reaction force measurement. In vivo measurements were used to evaluate computer models of the knee. Finite element models were used for contact analysis and for computing knee kinematics from measured knee forces. A third-generation system was developed for continuous monitoring of knee forces and kinematics outside the laboratory using a wearable data acquisition hardware. By using measured knee forces and knee flexion angle, we were able to compute femorotibial AP translation (-12 to +4 mm), mediolateral translation (-1 to 1.5 mm), axial rotation (-3° to 12°), and adduction-abduction (-1° to +1°). The neural-network-based classification system was able to identify walking, stair-climbing, sit-to-stand, and stand-to-sit activities with 100% accuracy. Our data may be used to improve existing in vitro models and wear simulators, and enhance prosthetic designs and biomaterials.

Sensitivity of knee soft-tissues to surgical technique in total knee arthroplasty

Restricted range of motion and excessive laxity are both potential complications of total knee arthroplasty (TKA). During TKA surgery, the surgeon is frequently faced with the question of how tightly to implant the prosthesis. The most common method of altering implantation tightness is to vary the thickness of the polyethylene inlay after the bone cuts have been made and the trial components inserted. We have sought to quantify how altering the polyethylene thickness may affect post-operative soft tissue tension for a range of prosthetic designs. Four different prosthetic designs were implanted into fresh-frozen cadaveric knee joints. All four designs were implanted in the standard manner, with a 100 Newton distraction force used to set soft tissue balance. The tibiofemoral force was then recorded at 15° intervals throughout the passive flexion range. After the standard implantation of each prosthesis, the tibial component was raised or lowered to mimic increasing and decreasing the polyethylene thickness by 2 mm and the force measurements repeated. Tibiofemoral force in extension correlated with implantation tightness for all prosthesis designs. Between 15° and 90°of knee flexion, all four designs were insensitive to changes in implantation tightness. Beyond 90° the effect was more notable in rotating platform mobile-bearing and cruciate-retaining prostheses than in posterior-stabilised mobile-bearing designs. The findings of this research may be useful in assisting surgical decision-making during the implantation of TKA prostheses.

Does saddle height affect patellofemoral and tibiofemoral forces during bicycling for rehabilitation?

The aim of the present study was to measure saddle height effects on knee joint load. Nine uninjured non-cyclists were evaluated in three saddle heights: 100% of trochanteric height-REF; 103% of REF-HIGH; and 97% of REF-LOW. Two-dimensional sagittal plane force applied on the pedal and kinematics were recorded. After inverse dynamics of the lower limb, knee resultant force was computed as tibiofemoral normal and shear components and compressive patellofemoral force. Peak patellofemoral compressive force and peak compressive and shear tibiofemoral forces did not differ when saddle height was changed. Knee angle at the lower crank position increased at LOW compared to REF and HIGH saddle height (p<0.02). Small saddle height changes (±3%) did not affect knee joint load, at low workloads on uninjured subjects, while changes in knee angle did not relate to effects on joint forces. These findings suggest that setting saddle height by knee angle secures the maintenance of joint load at low workloads on uninjured subjects.

Effects of Single-Bundle and Double-Bundle ACL Reconstruction on Tibiofemoral Forces and Joint Kinematics Under Dynamic Loading (SS-42)

Initial biomechanical studies have determined that the double-bundle technique significantly improved upon the single-bundle method: it better resists an externally applied moment and the stress distribution between the two grafts closely matches that of the normal ACL. However, there have not been any studies to evaluate the long-term effects of double-bundle ACL reconstruction on joint contact mechanics or articular cartilage integrity. Before this can be studied however, a direct measurement of joint contact forces must be examined. This study was conducted to establish a baseline relationship between ACL reconstructive techniques on joint kinematics and compressive forces. The purpose of this study was to compare the tibiofemoral joint kinematics and contact forces between single-bundle (SBR) and double-bundle (DBR) ACL reconstructive techniques during simulated squatting using a cadaver model. Twelve matched pairs of fresh frozen human cadaver knees (7 female and 3 male) with no evidence of knee pathology were stored at −20° C following harvest. The mean age of the specimens was 58 years (range 16 – 80). The specimens consisted of the distal third of the femur and the proximal two thirds of the tibia. Threaded rods (0.5-inch diameter Stainless Steel, McMaster-Carr, Los Angeles, CA) were inserted into the intramedullary canal of the femur and tibia and secured through an epoxy resin (Smooth-Cast 300, Smooth-On Inc, Easton, PA) for attachment to a test fixture which allows for dynamic simulated squatting. Additionally, the quadriceps tendon and semimembranosus and biceps femoris hamstrings were sutured to allow attachment to the test fixture for dynamic loading. An Optotrak Image Analysis System (Northern Digital; Waterloo, Ontario) with Optotrak Digitizer software (NDI 6D Architect) was used to track the position of the tibia with respect to the femur for kinematic measurements. K-Scan thin-film pressure sensors (4000; Maximum Pressure Range: 10,342 KPa, Sensing Area: 27.9 mm x 33 mm, Sensels per square cm: 62; Tekscan Inc, South Boston, MA) were used to simultaneously measure the tibiofemoral compressive forces in both compartments of the knee. The “I-scan” software program (Tekscan, Inc., South Boston, MA) was used to gather sensor data. Measurements were made in the intact knee and after the experimental procedure at each of 11 angles (0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100). For each outcome measure, two-way repeated measures analysis of variance was used to test for differences between the two experimental procedures with respect to the mean change from intact state. The model also included factors representing the effect of angle and an angle by procedure interaction. F-tests corresponding to simple effects were used to evaluate procedure differences in outcome within each angle when appropriate. One sample t-tests, which were based on the error term derived from the ANOVA, were used to evaluate significant change from intact state for each experimental procedure both across and within test angle. Analyses were performed using SAS statistical software Version 8. Statistical significance was based on p = 0.05. Rotation kinematics were different from the intact knee throughout the range of motion in the SBR group, while DBR was different at only high flexion angles while recreating proper rotation at low angles (p=0.004). In the medial compartment, both techniques successfully recreated joint contact forces (P=0.0006). In the lateral compartment, DBR failed to reapproximate native compressive forces throughout all flexion angles while SBR more closely reapproximated native compressive forces at high flexion angles. This study suggests compressive load is better restored with single-bundle than double-bundle ACL reconstruction. Joint kinematics are better restored with double-bundle reconstruction but still did not completely recreate the native ACL.

Assessment of isometricity before and after total knee arthroplasty: A cadaver study

Total knee arthroplasty (TKA) relies on soft tissue to regulate joint stability after surgery. In practice, the exact balance of the gaps can be difficult to measure, and various methods including intra-operative spreaders or distraction devices have been proposed. While individual ligament strain patterns have been measured, no data exist on the isometricity of the soft tissue envelope as a whole. In this study, a novel device was developed and validated to compare isometricity in the entire soft tissue envelope for both the intact and TKA knee.A spring-loaded rod was inserted in six cadaver knee joints between the tibial shaft and the tibial plateau or tibial tray after removing a 7 mm slice of bone. The displacement of the rod during passive flexion represented variation in tissue tension around the joint. The rod position in the intact knee remained within 1 mm of its initial position between 15° and 135° of flexion, and within 2 mm (±1.2 mm) throughout the entire range of motion (0–150°). After insertion of a mobile-bearing TKA, the rod was displaced a mean of 6 mm at 150° (p<0.001). The results were validated using a force transducer implanted in the tibial baseplate of the TKA, which showed increased tibiofemoral force in the parts of the flexion range where the rod was most displaced. The force measurements were highly correlated with the displacement pattern of the spring-loaded rod (r=−0.338; p=0.006).A simple device has been validated to measure isometricity in the soft tissue envelope around the knee joint. Isometricity measurements may be used in the future to improve implantation techniques during TKA surgery.

Erratum: Tibiofemoral force following total knee arthroplasty: Comparison of four prosthesis designs in vitro

AbstractThe authors have inadvertently left out several acknowledgments and wish to add them hereThis study was supported by DePuy International, the University of Western Australia, the Fremantle Hospital, and Mrs. Huggler.We would like to thank Dr. Hilaire Jacob for his help with the development of the mini‐force platform.We would also like to thank the Australian Orthopaedic Association (AOA) for the financial support of Dr. Jeffcote.

The variation in medial and lateral collateral ligament strain and tibiofemoral forces following changes in the flexion and extension gaps in total knee replacement

Achieving deep flexion after total knee replacement remains a challenge. In this study we compared the soft-tissue tension and tibiofemoral force in a mobile-bearing posterior cruciate ligament-sacrificing total knee replacement, using equal flexion and extension gaps, and with the gaps increased by 2 mm each. The tests were conducted during passive movement in five cadaver knees, and measurements of strain were made simultaneously in the collateral ligaments. The tibiofemoral force was measured using a customised mini-force plate in the tibial tray. Measurements of collateral ligament strain were not very sensitive to changes in the gap ratio, but tibiofemoral force measurements were. Tibiofemoral force was decreased by a mean of 40% (SD 10.7) after 90 degrees of knee flexion when the flexion gap was increased by 2 mm. Increasing the extension gap by 2 mm affected the force only in full extension. Because increasing the range of flexion after total knee replacement beyond 110 degrees is a widely-held goal, small increases in the flexion gap warrant further investigation.

Tibiofemoral force following total knee arthroplasty: Comparison of four prosthesis designs in vitro

Despite ongoing evolution in total knee arthroplasty (TKA) prosthesis design, restricted flexion continues to be common postoperatively. Compressive tibiofemoral force during flexion is generated through the interaction between soft tissues and prosthesis geometry. In this study, we compared the compressive tibiofemoral force in vitro of four commonly used prostheses: fixed-bearing PCL (posterior cruciate ligament)-retaining (PFC), mobile-bearing posterior-stabilized (PS), posterior-stabilized with a High Flex femoral component (HF), and mobile-bearing PCL-sacrificing (LCS). Fourteen fresh-frozen cadaver knee joints were tested in a passive motion rig, and tibiofemoral force measured using a modified tibial baseplate instrumented with six load cells. The implants without posterior stabilization displayed an exponential increase in force after 90 degrees of flexion, while PS implants maintained low force throughout the range of motion. The fixed-bearing PFC prosthesis displayed the highest peak force (214 +/- 68 N at 150 degrees flexion). Sacrifice of the PCL decreased the peak force to a level comparable with the LCS implant. The use of a PCL-substituting post and cam system reduced the peak force up to 78%, irrespective of whether it was a high-flex or a standard PS knee. However, other factors such as preoperative range of motion, knee joint kinematics, soft tissue impingement, and implantation technique play a role in postoperative knee function. The present study suggests that a posterior-stabilized TKA design might be advantageous in reducing soft tissue tension in deep flexion. Further research is necessary to fully understand all factors affecting knee flexion after TKA.

Sensitivity of the tibio-femoral response to finite element modeling parameters

A generic finite element (FE) model of the lower limb was used to study the knee response in-vivo during a one-legged hop. The approach uses an explicit FE code and a combination of estimated muscle forces and measured three-dimensional tibio-femoral kinematics and ground reaction force as input to the FE model. The sensitivity of the simulated tibio-femoral response to variations of key geometric and material parameters was investigated by performing a total of 38 different simulations. The amplitudes of both kinematic and kinetic responses were affected by the change of these parameters. For the current approach, the results suggest that while cartilage mechanical and geometric properties are very important for the estimation of tibio-femoral cartilage pressure, they have limited effects on the overall kinematic response. The study may help to better define the relative importance of modeling parameters for the development of subject-specific models.

E-Knee

Tibiofemoral forces determine polyethylene wear and affect the longevity of total knee prostheses. Previously, investigators relied on theoretic data from mathematical models to predict mechanical forces in the knee. Predictions of tibiofemoral forces are highly variable because of the complex interplay of the muscles involved in activities. Ideally, knee forces should be directly measured. An electronic total knee prosthesis (e-Knee) was developed to directly measure tibiofemoral compressive and tensile forces in vivo. After 13 years of research and development, the e-Knee was implanted into a patient in 2004. Tibiofemoral force data were collected intraoperatively and throughout the postoperative period during activities of daily living and during exercise. Direct measurement of knee forces can lead to a better understanding of the stresses seen following total knee arthroplasty. Information generated by the e-Knee will aid in the improvement of implant design and patient care.

A multiaxial force-sensing implantable tibial prosthesis

Accurate in vivo measurement of tibiofemoral forces is important in total knee arthroplasty. These forces determine polyethylene stresses and cold-flow, stress distribution in the implant, and stress transfer to the underlying implant bone interface. Theoretic estimates of tibiofemoral forces have varied widely depending on the mathematical models used. The six degrees of freedom of motion, complex articular surface topography, changing joint-contact position, intra- and extra-articular ligaments, number of muscles crossing the knee joint, and the presence of the patellofemoral joint contribute to the difficulty in developing reliable models of the knee. A prototype instrumented total knee replacement tibial prosthesis was designed, manufactured, and tested. This prosthesis accurately measured all six components of tibial forces ( R 2 > 0.997 ) . The prosthesis was also instrumented with an internal microtransmitter for wireless data transmission. Remote powering of the sealed implanted electronics was achieved using magnetic coil induction. This device can be used to validate existing models of the knee that estimate these forces or to develop more accurate models. In conjunction with kinematic data, accurate tibiofemoral force data may be used to design more effective knee-testing rigs and wear simulators. Additional uses are intraoperative measurement of forces to determine soft-tissue balancing and to evaluate the effects of rehabilitation, external bracing, and athletic activities, and activities of daily living.

An implantable telemetry device to measure intra-articular tibial forces

Tibial forces are important because they determine polyethylene wear, stress distribution in the implant, and stress transfer to underlying bone. Theoretic estimates of tibiofemoral forces have varied between three and six times the body weight depending on the mathematical models used and the type of activity analyzed. An implantable telemetry system was therefore developed to directly measure tibiofemoral compressive forces. This system was tested in a cadaver knee in a dynamic knee rig. A total knee tibial arthroplasty prosthesis was instrumented with four force transducers located at the four corners of the tibial tray. These transducers measured the total compressive forces on the tibial tray and the location of the center of pressure. A microprocessor performed analog-to-digital signal conversion and performed pulse code modulation of a surface acoustic wave radio frequency oscillator. This signal was then transmitted through a single pin hermetic feed-through tantalum wire antenna located at the tip of the stem. The radio frequency signal was received by an external antenna connected to a receiver and to a computer for data acquisition. The prosthesis was powered by external coil induction. The tibial transducer accurately measured both the magnitude and the location of precisely applied external loads. Successful transmission of the radio frequency signal up to a range of 3 m was achieved through cadaveric bone, bone cement, and soft tissue. Reasonable accuracy was obtained in measuring loads applied through a polyethylene insert. The implant was also able to detect unicondylar loading with liftoff.