While fluoroscopic techniques have been widely utilized to study in vivo kinematic behavior of total knee arthroplasties, determination of the contact forces of large population sizes has proven a challenge to the biomedical engineering community. This investigation utilizes computational modeling to predict these forces and validates these with independent telemetric data for multiple patients, implants, and activities. Two patients with telemetric implants, the first of which was studied twice with the reexamination occurring 8 years after the first, were studied. Three-dimensional models of the patients' bones were segmented from CT and aligned with the design models of the telemetric implants. Fluoroscopy was collected for gait, deep knee bend, chair rise, and stair activities while being synchronized to the ground reaction force (GRF) plate, telemetric forces, knee flexion angles, electromyography (EMG), and vibration sensors. Registration of the implants and bones to the 2-D fluoroscopy provided the 6 degree of freedom kinematic data for each object. Orientation and position of the components, the GRFs, ligament properties, and muscle attachment locations were the only inputs to the Kane's dynamics inverse solution. Dynamic contact mapping and pseudo-inverse solution method were incorporated to output the predicted muscle forces of the vastus lateralis, rectus femoris, vastus medialis, biceps femoris long head, and gastrocnemius and contact forces at the patellofemoral and medial and lateral tibiofemoral. While every major muscle of the lower limb was incorporated into the model, these five were used in the validation process. EMG signals were processed to determine the neural excitation, muscle activation, and using the dynamic muscle length from the kinematics, the tension generated by these muscles.Introduction
Methods
Previous fluoroscopy studies have been conducted on numerous primary-type TKA, but minimal in vivo data has been documented for subjects implanted with revision TKA. If a subject requires a revision TKA, most often the ligament structures at the knee are compromised and stability of the joint is of great concern. In this present study, subjects implanted with a fixed or mobile bearing TC3 TKA are analyzed to determine if either provides the patient with a significant kinematic advantage. Ten subjects are analyzed implanted with fixed bearing PFC TC3 TKA and 10 subjects with a mobile bearing PFC TC3 TKA. Each subject underwent a fluoroscopic analysis during four weight bearing activities: deep knee bend (DKB), chair rise, gait, and stair descent. Fluoroscopic images were taken in the sagittal plane at 10 degree increments for the DKB, 30 degree increments for chair rise, and at heel strike, toe off, 33% and 66% cycle gait and stair descent.Introduction
Methods
Electromyography (EMG) is the best known method in obtaining in vivo muscle activation signals during dynamic activities, and this study focuses on comparing the EMG signals of the quadriceps muscles for different TKA designs and normal knees during maximum weight bearing flexion. It is hypothesized that the activation levels will be higher for the TKA groups than the normal group. Twenty-five subjects were involved in the study with 11 having a normal knee, five a rotating platform (RP) posterior stabilized (PS) TKA, and nine subjects with a PFC TC3 revision TKA. EMG signals were obtained from the rectus femoris, vastus medialis, and vastus lateralis as the patients performed a deep knee bend from full extension to maximum flexion. The data was synchronized with the activity so that the EMG data could be set in flexion-space and compared across the groups. EMG signals were pre-processed by converting the raw signals into neural excitations and normalizing this data with the maximum voluntary contraction (MVC) performed by the subject. The signals were then processed to find the muscle activations which, normalized by MVC, range from 0 to 1.Introduction
Methods
Orthopaedic companies spend years and millions of dollars developing and verifying new total knee arthroplasty (TKA) designs. Recently, computational models have been used in the hopes of increasing the efficiency of the design process. The most popular predictive models simulate a cadaveric rig. Simulations of these rigs, although useful, do not predict in vivo behavior. Therefore, in this current study, the development of a physiological forward solution, or predictive, rigid body model of the knee is described. The models simulate a non-weight bearing extension activity or a weight-bearing deep knee bend (DKB) activity. They solve for both joint forces and kinematics simultaneously and were developed from the ground up. The models are rigid body and use Kane's dynamical equations. The model began with a simple two dimensional non-weight bearing extension activity model of the tibiofemoral joint. Step by step the model was expanded. Quadriceps and hamstring muscles were added to drive the motion. Ligaments were added represented by multiple non-linear spring elements. The model was expanded to three-dimensions (3D) allowing out of plane motions and calculation of medial and lateral condylar forces. The patella was added as its own body allowing for simulation of the patellofemoral joint. The model was then converted to a weight bearing deep knee bend activity. A pelvis and trunk were added and muscles were given physiological origin and insertion points. A modified proportional-integral-derivative (PID) controller was implemented to control the rate of flexion and also to assist in joint stability by adjusting the force in individual quadriceps muscles. A method for representing articulating geometry was developed. Once the deep knee bend model was fully developed (Figure 1) it was converted back to a non-weight bearing extension model (Figure 2) resulting in simulations of a normal knee performing a weight bearing and non-weight bearing activity. The tibiofemoral kinematic results were compared to in vivo kinematics obtained from a fluoroscopy study of five normal subjects. Parameters from the CT models of one of these subjects (Subject 3) were used in the model. The model kinematics behave as the normal knee does in vivo. The kinetic results were within reasonable ranges with a maximum total quadriceps force of 0.86 BW and 4.73 BW for extension and DKB simulations, respectively (Figure 3 and Figure 4). The maximum total tibiofemoral forces were 1.26 BW and 3.70 BW for extension and DKB, respectively. The relationship between the quadriceps force, patella ligament force and patellofemoral forces are consistent with how the extensor mechanism behaves (Figure 3 and Figure 4). The patellofemoral forces are low between 0 and 20 degrees flexion and the patella ligament and quadriceps forces are close in magnitude from 0 to around 70 degrees flexion when the patellofemoral forces increase and the quadriceps forces increase relative to the patella ligament force. The model allows for virtual implantation of TKA geometry and after kinematic and kinetic validation from in vivo TKA data can be used to predict the behavior of TKA in vivo.
Multiple video fluoroscopic analyses have been performed to determine the in vivo kinematic patterns of total knee arthroplasty (TKA) and non implanted knees. Unfortunately, many of these studies were not correlated with bearing surface forces and possible failure modes that could be detected with a sound sensor. Therefore, the objective of the present study was to conduct a comparative analysis of the kinematic data derived for all subjects having a TKA who were analyzed over the past seventeen years at our laboratory and to determine how these patterns correlate with bearing surface forces and joint sound. Initially, femorotibial contact positions and axial rotation magnitudes were derived for subjects having either a non implanted or implanted knee. Non implanted knees consisted of normal and anterior cruciate ligament (ACL) deficient knees (ACLD). Implanted knees consisted of posterior stabilized (PS) fixed (PSF) and mobile (PSM) bearing, posterior cruciate ligament retaining (PCR) fixed (PCRF) and mobile (PCRM) bearing, posterior cruciate sacrificing mobile (PCSM) bearing and ACL retaining fixed (ACRF) bearing TKA. Each subject, while under fluoroscopic surveillance, performed a weight-bearing deep knee bend and/or normal gait. Using a three-dimensional (3D) model fitting approach, the relative pose of knee implant components were determined in 3D from a single-perspective fluoroscopic image by manipulating a CAD model in three-dimensional space. Anterior/posterior (A/P) contact positions for both the medial and lateral condyles and axial rotation of the femoral component relative to the tibial component were assessed. Then, a subset of these subjects were further analyzed to determine their in vivo bearing surface forces and joint sound using a more recently derived protocol for analyzing audible signals.INTRODUCTION
METHODS
Commercial C-arm fluoroscopes are routinely used to analyze human skeletal joints during motions such as deep knee bends, or chair rises. Such diagnostics are used to characterize pre and post operative arthoplasty results, particularly in association with total joint replacement procedures. Stationary fluoroscopes restrict the patient motion and load conditions, thus diminishing the diagnostic utility of the results. A new class of fluoroscopy has been developed in which a robotic mechanization is used to allow selected joints to be x-rayed while the human subjects perform natural motions such as walking. The tracking fluoroscope system (TFS) is a mobile robot that acquires real-time x-ray records of hip, knee, or ankle joint motion while the patient walks normally. Because the fluoroscope line of sight dynamically tracks the joint of interest, the TFS provides clearer and contained joint images. The technical features of the TFS will be reviewed, recent development testing summarized, and the results of preliminary patient trials presented.
Previous fluoroscopic studies compared total knee arthroplasty (TKA) kinematics to normal knees. It was our hypothesis that comparing TKA directly to its non-replaced controlateral knee may provide more realistic kinematics information. Using fluoroscopic analysis, we aimed to compare knee flexion angles, femoral roll-back, patellar tracking and internal and external rotation of the tibia. 15 patients (12 women and 3 men) with a mean age of 71.8 years (SD=7.4) operated by the same surgeon were included in this fluoroscopic study. For each patient at a minimum one year after mobile-bearing TKA, kinematics of the TKA was compared to the controlateral knee during three standardized activities: weight-bearing deep-knee bend, stair climbing and walking. A history of trauma, pain, instability or infection on the non-replaced knee was an exclusion criteria. A CT-scan of the non-replaced knee was performed for each patient to obtain a 3-D model of the knee. The Knee Osteoarthitis Outcome Score (KOOS) was also recorded.Introduction
Material and methods
Posterior stabilized (PS) total knee arthroplasty (TKA) provides posterior stability with the use of a cam-post mechanism which performs the function of the posterior cruciate ligament. The tibial post engages with the femoral cam, prevents the femur from sliding anteriorly and provides the posterior femoral rollback necessary for achieving deep flexion of the knee. However, these designs do not substitute the resection of the anterior cruciate ligament. In order to overcome this deficit, other TKA designs have been recently introduced to provide dual support, with the help of dual cam-post engagement mechanism. Various studies conducted on the PS TKA have suggested that the cam-post mechanism does not engage as designed, resulting in tibial post wear and increased stresses resulting in backside wear of the polyethylene insert component. Also, the in vivo data pertaining to the actual cam-post engagement mechanism in bi-cruciate stabilized knees is still very limited. Therefore, the objective of this study was to determine the cam-post mechanism interaction under in vivo, weight bearing conditions for subjects implanted with either a Rotating Platform (RP) Posterior Stabilized (PS) TKA or a bi-cruciate stabilizing TKA (BCS). In-vivo, weight-bearing, 3D knee kinematics were determined for eight subjects (9 knees) having a RP-PS TKA (DePuy Inc.) and eight subjects (10 knees) having BCS TKA (Smith&Nephew Inc.), while performing a deep knee bend. 3D kinematics was recreated from the fluoroscopic images using a previously published 3D-to-2D registration technique (Figure 1). Images from full extension to maximum flexion were analyzed at 10° intervals. Once the 3D kinematics of all implant components was recreated, the cam-post mechanism was scrutinized. The distance between the interacting surfaces was monitored throughout the flexion and the predicted contact map was calculated. The instances, when the minimum distance between the cam and post surfaces dropped to zero was considered to indicate the engagement of the mechanism. This analysis was carried out for both the, anterior and posterior cam-post engagement sites.INTRODUCTION
METHODS
Knee simulators are being used to evaluate wear. The current international standards have been developed from clinical investigations of the normal knee [1, 2] or from a single TKA patient [3, 4]. However, the forces and motions in a TKA patient differ from a normal knee and, furthermore, the resulting kinematic outcomes after TKA will depend on the design of the device [5]. Consequently, these standard tests may not recreate in-vivo conditions; therefore, the goal of this study was to perform a novel wear simulation using design-specific inputs that have been derived from fluoroscopic images of a deep knee bend. A wear simulation was developed using fluoroscopic data from a pool of eighteen TKA patients performing a deep knee bend. All patients had a Sigma CR Fixed Bearing implant (DePuy) and were well functioning (Knee Society Score > 90). A single patient was selected that represented the typical motions, which was characterized by early rollback followed by anterior motion with an overall modest internal tibial rotation (Figure 1). The relative motion between the femoral and tibial components was transformed to match the coordinate system of an AMTI knee wear simulator [6] and a compressive load input was derived using inverse dynamics [7]. The resulting force and motions (Figure 2) were then applied in a wear simulation with 5 MRad crosslinked and remelted polyethylene for 3 Mcyc at 1 Hz. Components were carefully positioned and each joint (n=3) was tested in 25% bovine calf serum (Hyclone Laboratories), which was recirculated at 37±2°C [3]. Serum was supplemented with sodium azide and EDTA. Wear was quantified gravimetrically every 0.5 Mcyc using a digital balance (XP250, Mettler-Toledo) with load soak compensation.INTRODUCTION
METHODS
Telemetric implants have provided us with invaluable data as to the in vivo forces occurring in implanted knee joints. However, only a few of them exists. The knee is one of the most studied joints in the human body and various mathematical knee models have been used in the past to predict forces. However, these simulation studies have also been carried out on a small group of patients limiting their general usefulness in understanding overall trends of knee behavior. Therefore, it is the purpose of this research to study the implant forces experienced by a large group of patients so as to have a better understanding of the overall magnitudes and their variability with knee flexion. The patients were selected from a large database of over 3000 knees for which kinematic analysis had previously been carried out using fluoroscopy. The criteria used for selection was that the patients had a successful knee implant (HSS >90) and were able to perform a weight bearing deep knee bend of at least 110 degrees. The patients were randomly chosen without any other restrictions. The kinetic analysis was carried on a cohort of over 100 patients using a previously published inverse dynamic rigid body model. This model, which has been validated using telemetric data, is capable of predicting the contact forces on the medial and lateral condyles of the knee. Analysis was carried out till 130 degrees of flexion to remove any effect of thigh calf contact that the model does not incorporate. 20 normal knees were also included for comparison.INTRODUCTION
METHODS
Anterior knee pain is one of the most frequently reported musculoskeletal complaints in all age groups. However, patient's complaints are often nonspecific, leading to difficulty in properly diagnosing the condition. One of the causes of pain is the degeneration of the articular cartilage. As the cartilage deteriorates, its ability to distribute the joint reaction forces decreases and the stresses may exceed the pain threshold. Unfortunately, the assessment of the cartilage condition is often limited to a detailed interview with the patient, careful physical examination and x-ray imaging. The X-ray screening may reveal bone degeneration, but does not carry sufficient information of the soft tissues' conditions. More advanced imaging tools such as MRI or CT are available, but these are expensive, time consuming and are only suitable for detection of advanced arthritis. Arthroscopic surgery is often the only reliable option, however due to its semi-invasive nature, it cannot be considered as a practical diagnostic tool. However, as the articular cartilage degenerates, the surfaces become rougher, they produce higher vibrations than smooth surfaces due to higher friction during the interaction. Therefore, it was proposed to detect vibrations non-invasively using accelerometers, and evaluate the signals for their potential diagnostic applications. Vibration data was collected for 75 subjects; 23 healthy and 52 subjects suffering from knee arthritis. The study was approved by the IRB and an Informed Consent was obtained prior to data collection. Five accelerometers were attached to skin around the knee joint (at the patella, medial and lateral femoral condyles, tibial tuberosity and medial tibial plateau). Each subject performed 5 activities; (1) flexion-extension, (2) deep knee bend, (3) chair rising, (4) stair climbing and (5) stair descent. The vibration and motion components of the signals were separated by a high pass filter. Next, 33 parameters of the signals were calculated and evaluated for their discrimination effectiveness (Figure 1). Finally the pattern recognition method based on Baysian classification theorem was used for classify each signal to either healthy or arthritic group, assuming equal prior probabilities. The variance and mean of the vibration signals were significantly higher in the arthritic group (p=2.8e-7 and p=3.7e-14, respectively), which confirms the general hypothesis that the vibration magnitudes increase as the cartilage degenerates. Other signal features providing good discrimination included the 99th quantile, the integral of the vibration signal envelope, and the product of the signal envelope and the activity duration. The pattern classification yielded excellent results with the success rate of up to 92.2% using only 2 features, up to 94.8% using 3 (Figure 2), and 96.1% using 4 features. The current study proved that the vibrations can be studied non-invasively using a low-cost technology. The results confirmed the hypothesis that the degeneration of the cartilage increases the vibration of the articulating bones. The classification rate obtained in the study is very encouraging, providing over 96% accuracy. The presented technology has certainly a potential of being used as an additional screening methodology enhancing the assessment of the articular cartilage condition.
Total shoulder arthroplasty (TSA) implants are used to restore function to individuals whose shoulder motions are impaired by osteoarthritis. To improve TSA implant designs, it is crucial to understand the kinematics of healthy, osteoarthritic (OA), and post-TSA shoulders. Hence, this study will determine in vivo kinematic trends of the glenohumeral joints of healthy, OA, and post-TSA shoulders. In vivo shoulder kinematics were determined pre and post-operatively for five unilateral TSA subjects with one healthy and a contralateral OA glenohumeral joint. Fluoroscopic examinations were performed for all three shoulder categories (healthy, OA, and post-TSA) for each subject shoulder abduction and external rotation. Then, three-dimensional (3D) models of the left and right scapula and humerus were constructed using CT scans. For post-operative shoulders, 3D computer-aided design models of the implants were obtained. Next, the 3D glenohumeral joint kinematics were determined using a previously published 3D to 2D registration technique. After determining kinematics, relative Euler rotation angles between the humerus and scapula were calculated in MATLAB® to determine range of motion (ROM) and kinematic profiles for all three shoulder categories. The ROMs for each category were compared using paired t-tests for each exercise. Also, the location of the contact point of the humerus on the glenoid was found. This allowed the vertical translation from the most superior to most inferior contact point (SI contact range) to be calculated as well as the horizontal translation from the most anterior to most posterior contact point (AP contact range). The SI and AP contact ranges for all shoulder categories were compared using paired t-tests for each exercise.INTRODUCTION
Methods
Achieving high flexion after total knee arthroplasty is very important for patients in Asian countries where deep flexion activities are an important part of daily life. The Bi-Surface Total Knee System (Japan Medical Material, Kyoto, Japan), which has a unique ball-and-socket mechanism in the mid-posterior portion of the femoral and tibial components, was designed to improve deep knee flexion and long-term durability after total knee arthroplasty (Figure 1). The purpose of this study was to determine the in vivo three dimensional kinematics of Bi-Surface Total Knee System in order to evaluate and analyze the performance of this system with other conventional TKA designs currently available in the market today. Three dimensional kinematics were evaluated during a weight-bearing deep knee bend activity using fluoroscopy and a 2D-to-3D registration technique for 66 TKA. Each knee was analyzed to determine femorotibial kinematics, including weight-bearing range of motion, anterior/posterior contact position, and tibio-femoral rotation.Introduction
Materials and Methods
Numerous studies have been conducted to investigate the kinematics of the lumbar spine, and while many have documented its intricacies, few have analyzed the complex coupled out-of-plane rotations inherent in the low back. Some studies have suggested a possible relationship between patients having low back pain (LBP) or degenerative conditions in the lumbar region and various degrees of restricted, excessive, or poorly-controlled lumbar motion. Conversely, others in the orthopedic community maintain there has been no distinct correlation found between spinal mobility and clinical symptoms. The objective of this study was to evaluate both the in-plane and coupled out-of-plane rotational magnitudes about all three motion axes in both symptomatic and asymptomatic patients. Ten healthy, 10 LBP, and 10 degenerative patients were CT scanned and evaluated under fluoroscopic surveillance while performing flexion/extension of the lumbar spine. Three-dimensional, patient-specific bone models were created and registered to fluoroscopic images using a 3D-to-2D model fitting algorithm. Introduction
Methods
Subjects having a posterior cruciate ligament sacrificing (PCLS) mobile bearing TKA seem to experience less translation during gait, but often achieve less weight-bearing flexion. More recently, posterior stabilisation has been added to PCLS mobile bearing TKA, hoping to increase flexion. Therefore, the objective of this multi-center study was to determine the in vivo kinematics for subjects implanted with a mobile bearing PS TKA that attempts to maintain high contact area. Subjects with 10 TKA from 2 surgeons were asked to perform maximum weight-bearing flexion (deep knee bend (DKB)) and gait while under fluoroscopic surveillance. During weight bearing flexion, the 3-D kinematics of the TKA were determined by analyzing fluoroscopic images in the sagittal plane at 30 degree increments. Fluoroscopic images taken in the frontal plane from four increments during the stance phase of gait were analyzed. The average weight-bearing flexion was 116 degrees and the average medial and lateral anteriorposterior (AP) translation was posterior with −1.9 mm and −5.4 mm, respectively, from full extension to maximum weight-bearing flexion. The average femorotibial axial rotation from full extension to maximum weight-bearing flexion was 3.9 degrees. During the stance phase of treadmill gait, patients experienced 0.8 mm (0.1 mm to 2.3 mm, SD=0.8 mm) of “pure” mediolateral translation of the femur relative to the tibia. The femorotibial axial rotation was 4.6 degrees from heel-strike to toe-off (Table 3). The posterior femoral rollback and axial rotation patterns were similar to the normal knee, albeit experiencing less overall motion. More noticeably, subjects in this study experienced a significantly greater weight-bearing flexion than previous subjects analyzed with a mobile bearing PCLS TKA and more reproducible “fan-like” patterns, where the lateral condyle rolled greater posteriorly than the medial condyle.
Recent fluoroscopic analyses evaluating the kinematic function of TKAs have demonstrated significant variability among patients with identical implant designs, suggesting surgical technique also influences function. To help explain these kinematic variations, we used intraoperative compartment pressure sensors to assess balancing at trial reduction and ROM then correlated these intraoperative findings with patients’ postoperative kinematics, assessed using video fluoroscopy. This study involved 16 patients implanted with a posterior cruciate-sacrificing LCS TKA using a balanced gap technique. After releases in extension, the femur was rotated the appropriate amount to create a rectangular flexion gap relative to the cut tibial surface. As the knee was taken through a ROM from 0–120°, the sensors (placed on the tibial insert trial) dynamically measured the magnitude and location of compartment pressures throughout the ROM. Six to nine months postoperatively, all patients performed successive weight-bearing deep knee bends to maximum flexion under fluoroscopic surveillance. Each patient’s femoro-tibial contact positions and liftoff values were compared to their respective intraoperative compartment pressure findings to establish correlations. Fluoroscopic results correlated closely with intraoperative compartment pressures and balance data. Three of the 16 patients had condylar liftoff: two patients experienced liftoff in flexion and one in extension (medial). The patient who experienced medial liftoff in extension had decreased medial compartment pressure and a slight valgus malalignment (7° of anatomic alignment). Two of the 13 patients without liftoff had abnormal compartment pressures in extension. In both cases, mechanical axis alignment resulted in loading of the lax compartment with weight-bearing. The other 11 patients had normal compartment pressures in extension and no condylar liftoff. One of these patients had slight valgus (7°) and another slight varus malalignment (4°), but both had normal compartment pressures. Despite good compartment balance, average tibiofemoral rotation was inadequate; three of 16 patients experienced opposite axial rotation with flexion. Extensive ligament release did not always result in equal compartment pressure magnitudes and distributions; compartment balance was influenced by the nature of the release. These data suggest that liftoff may require both a compartment pressure imbalance and abnormal alignment that together exacerbate the laxity with physiologic loading. Previous kinematic studies of LCS knees have shown that the balanced gap technique produces wellbalanced compartment pressures, resulting in TKAs with little lift-off and very good translational and rotational characteristics. Therefore, while a given implant design may have inherent kinematic tendencies, surgical technique may significantly impact kinematic performance. To optimize implant kinematics and subsequent TKA function and longevity, it may be important for surgeons to accurately balance the flexion and extension gaps. Characteristic compartment pressure patterns and distributions for various ligament releases may shed some light on less than optimal rotational kinematic performance.
All over the world, obesity rates are on the rise. Medical complications and increased health risks are often associated with being overweight or obese, but a thorough understanding of in vivo motions for obese, overweight and normal weight subjects does not exist. Therefore, the objective of this study was to compare knee kinematics in TKA subjects by body mass index (BMI). In vivo knee kinematics were determined for 253 TKA subjects during a Deep Knee Bend (DKB) from full extension to maximum flexion using a 3D to 2D image registration technique. Each of these subjects was then classified into one of three BMI categories: obese (BMI greater than or equal to 30), overweight (BMI greater than or equal to 25 and less than 30) and normal weight (BMI less than 25 and greater than or equal to 18.5). Subjects were provided by 11 surgeons using ten different TKA devices. All subjects were deemed clinically successful. On average, weight bearing range of motion (ROM) for the obese (n=79), overweight (n=113) and normal weight (n=61) groups were 107.7° (range: 74° to 136°, standard deviation (σ) =14.9°), 109.6° (60° to 150°, σ=17.5°) and 114.1° (72° to 147°, σ=14.4), respectively. ROM of 90° or less was seen in 16.5% of the obese subjects, 14.2% of the overweigh subjects and 6.6% of the normal weight subjects. ROM of 125° or more was seen in 15.2% of the obese subjects, 16.8% of the overweight subjects and 23.0% of the normal weight subjects. From full extension to maximum flexion the obese, overweight and normal weight groups averaged 8.65° (−5.14° to 22.51°, σ=6.22°), 7.58° (−2.85° to 24.72°, σ=5.71°) and 5.72° (−4.84° to 19.43°, σ=5.65°) of axial rotation. Axial rotation of 3° or less was seen in 20.25% of the obese subjects, 23.01% of the overweight subjects and 39.34% of the normal weight subjects. Axial rotation of greater than 9° was seen in 51.90% of the obese subjects, 35.40% of the overweight subjects and 26.23% of the normal weight subjects. Opposite axial rotation was seen in 8.86% of the subjects in the obese group, 9.73% of the overweight group and 9.84% of the normal weight group. On average, from full extension to maximum flexion, the medial condyle for the obese, overweight and normal weight groups experienced −5.44mm (−22.20mm to 8.04mm, σ=7.9mm), −6.30mm (−25.22mm to 5.35mm, σ=7.36mm) and −4.78mm (−20.79mm to 5.49mm, σ=6.68mm) of posterior femoral rollback (PFR), respectively. The obese, overweight and normal weight groups averaged −12.66 mm (−34.57mm to 0.34mm, σ=9.32mm), −12.38mm (−36.72mm to 1.83mm, σ=10.33mm) and −9.39 mm (−34.55mm to 0.35mm, σ=8.98mm) of lateral PFR, respectively. Condylar lift-off of greater than 1mm was seen in 16.46% of obese subjects, 10.62% of overweight subjects and 11.48% of normal weight subjects. Various statistical differences were seen across the groups. The normal weight subjects had significantly higher ROM that the obese subjects (p=0.0184), while there was no difference seen between the normal weight and overweight groups or the overweight and obese groups. The obese and the overweight groups had significantly more axial rotation than the normal weight group from 0° to 90°, 0° to maximum flexion, 30° to 90°, 30° to maximum flexion and 60° to 90°. There were a significantly higher number of cases of condylar lift-off for obese subjects when compared to both normal weight and overweight groups. It can be concluded that body mass index does play a factor in TKA kinematics.
In vivo kinematic analyses of total hip arthroplasty (THA) have determined femoral head separation from the medial aspect of the acetabular component can occur. Various bearing materials are currently used in THA today. The objective of this study was to determine if differences in the incidence and magnitude of femoral head separation exist among various bearing surfaces for THA during different weight-bearing activities. 205 clinically successful subjects implanted with either metal-on-metal (MOM), metalon-polyethylene (MOP), ceramic-on-ceramic (COC) or ceramic-on-polyethylene (COP) materials were analyzed using video-fluoroscopy. Each patient performed either gait on a treadmill or an abduction-adduction activity. The fluoroscopic information was then analyzed using a computer aided 3D model fitting technique to determine the incidence and magnitude of hip separation. Additional variables analyzed included femoral head diameter, follow-up duration, and type of surgical approach utilized. Less separation was noted with increasing femoral head diameter during abductionadduction. Increased separation was observed during gait as follow-up duration increased. Hip separation was greater during gait when a posterolateral surgical approach was used but was greater in abduction-adduction if a antero-lateral approach was selected. The incidence and magnitude of hip separation during gait was least in subjects with COC THA and least with COC and MOM THA when analyzed during abduction-adduction. It’s been proposed that THA patients are subject to femoral head separation due to alterations in the soft tissue supporting structures during THA that affect constraint of the joint. The current analysis demonstrates lower magnitudes and incidence of THA separation occur when hard-on-hard bearing surfaces are selected and can vary based on femoral head diameter, follow-up duration, and surgical approach used. Potential detrimental effects resulting from THA separation include premature polyethylene wear, component loosening (secondary to impulse loading conditions) and hip instability.
Previous in vivo studies pertaining to THA performance have focused on the analysis of gait. Unfortunately, higher demand activities have not yet been analyzed. Therefore, the objective of the present study was to determine the in vivo kinematics for THA patients, using fluoroscopy, while they performed four higher demand activities. The 3D in vivo kinematics of 10 THA patients were analyzed during the following activities: pivoting (PI), tying a shoe (SHOE), sitting down (SDOWN) and standing up (SUP) with and without the aid of handrails. Patients were matched for age, height, weight, body mass index, diagnosis and femoral head diameter to control for confounding variables possibly having influence on the hip performance and kinematics of the various activities. The largest amount, incidence and variation of separation (femoral head sliding in the acetabular cup) were achieved during the PI with 1.5mm (SD 1.1) and 9 of 10 (90%) subjects experiencing separation. For the SHOE, SDOWN and SUP activities the average separation values were 1.1, 1.2 and 0.7mm, respectively. Femoral head separation was observed in 8 of 10 subjects (80%) during SHOE, in 9 (90%) during SDOWN, and in only one of 6 (60%) during SUP. In this present study, subjects demonstrated hip separation during the high demand subjects, which could be a concern because these same activities are subjected to higher bearing surface forces. Also, the presence of hip separation leads to reduced contact area between the femoral head and the acetabular cup, possibly leading to higher contact stresses.
Low back pain (LBP) in the region of the lumbar spine is a significant problem among individuals, and efforts focused on treating both the symptoms and causes of LBP have proven to be difficult. Aside from conservative treatments, the predominant surgical approach for treating degenerative spine conditions has been to fuse the vertebral bodies at the symptomatic level. Even today, surgical fusion and its effect on adjacent levels are still not fully understood. Therefore, the objective of this study was to use fluoroscopy and mathematical modeling techniques to identify the in vivo kinematics and kinetics in subjects having either a normal, degenerative or fused condition of the lumbar spine. Twenty-five subjects (ten normal, ten degenerative, and five fusion) were evaluated under fluoroscopic surveillance while performing flexion/extension of the lumbar spine. Subjects within the normal and degenerative groups were analyzed only once, while subjects from the fusion group were analyzed both pre-operatively and at a minimum of six months post-operative. The fusion group consisted of three subjects symptomatic at L4/L5, with the remaining two subjects symptomatic at L5/S1. In vivo kinematics data were derived using a 3D-to-2D model fitting algorithm and served as input into a 3D mathematical model of the lumbar spine. The parametric, inverse dynamics mathematical model was created to allow for the determination of the bearing surface contact and muscle forces at each level of the lumbar spine. Three-dimensional kinematics analyses revealed that subjects classified as having a normal lumbar spine experienced a more uniform motion pattern compared to those observed in the degenerative and fusion groups. Alternatively, the degenerative and fusion subjects demonstrated a more coupled motion pattern in order to perform in plane flexion/extension. Compared to the normal group, rotations in the sagital plane decreased by an average of 28% at the pathological level in the degenerative group, while in the fusion group segmental motions slightly increased at the adjacent levels. Results from the mathematical model also revealed higher out-of-plane forces and increased loading at symptomatic and adjacent levels in both the degenerative and fused groups compared to forces observed in the normal spine. The abnormal motion patterns, which result from decreased or loss of motion at pathological levels in the degenerative and fusion groups, are believed to result in higher resultant forces in the spine. This may be subjecting the intervertebral discs to increased stresses, and as a consequence may be linked to more rapid degeneration at levels where the abnormal kinematics are occurring.
