Jacobus H. Müller

Differential Forces Within the Proximal Patellar Tendon as an Explanation for the Characteristic Lesion of Patellar Tendinopathy: An In Vivo Descriptive Experimental Study

Background Patellar tendinopathy is a common condition affecting the posterior region of the proximal patellar tendon, but the reason for this typical location remains unclear. Hypothesis The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. Study Design Descriptive laboratory study. Method An optic fiber technique was used to detect forces in both the anterior and the posterior regions of the proximal patellar tendon in 7 healthy persons. The optic fiber force sensor works on the principle of the amplitude modulation of transmitted light when the optic fiber is geometrically altered owing to the forces acting on it. Longitudinal strain in the tendon or ligament produces a negative transverse strain, thus causing a force that effectively squeezes the optic fiber. Measurements were recorded during the following exercises: closed kinetic chain quadriceps contraction (eccentric and concentric), open kinetic chain quadriceps contraction (eccentric and concentric), a step exercise, and a jump exercise. Results During all the exercises, the peak differential signal output in the posterior location of the proximal patellar tendon was greater than in the corresponding anterior location. The greatest differential signal output was found in the jump and squat exercises. Conclusion The posterior region of the proximal patellar tendon is subjected to greater tendinous forces than is the corresponding anterior region. This finding supports the tensile-overload theory of patellar tendinopathy. Clinical Relevance Jump activities and deep squat exercises expose the patellar tendon to very large tendinous forces.

A Novel Complimentary Filter for Tracking Hip Angles During Cycling Using Wireless Inertial Sensors and Dynamic Acceleration Estimation

As wireless motion sensors become more compact and robust, new opportunities emerge to develop wearable measurement technologies for in-field sports analysis. This paper presents a nonlinear complimentary filter for tracking 3-D hip joint angles during cycling using inertial and magnetic measurement systems (IMMSs). The filter utilizes a novel method of dynamic acceleration compensation in the sensor frame based on the assumption of pendulum motion of the thigh around the hip joint center. A dynamic calibration is proposed in which the center of rotation of the thigh IMMS can be estimated during a functional hip movement in standing. Validation results from a gold-standard optical system showed that the filter IMMS tracking is drift-free with mean absolute errors of less than 3° for all IMMS axes combined at low, medium, and high pedaling speeds. Hip angles were also validated using the Vicon biomechanical model for standing and sitting calibration poses as well as true and normalized soft-tissue-artefact (STA). The best mean absolute errors for the sagittal, frontal, and coronal planes were 0.8°, 6.7°, and 2.2°, respectively. Variability due to calibrations and STA ranged from 1.4° to 8.1°. This demonstrates the high accuracies possible for IMMS tracking using algorithms designed for specific sports despite larger errors due to modeling.

A Complementary Filter for Tracking Bicycle Crank Angles Using Inertial Sensors, Kinematic Constraints, and Vertical Acceleration Updates

In-field tracking of crank angles is important for analyzing outdoor cycling biomechanics, but current encoder-based methods are expensive and time-consuming. Inertial and magnetic measurement systems (IMMSs) have the potential for minimally invasive crank angle tracking, although errors due to magnetic interference and static calibration hinder performance. This paper presents a nonlinear complimentary filter, called the constrained rotational acceleration and kinematics (CRANK) filter, which estimates crank angles without magnetometer measurements or a static calibration for the crank arm IMMS. The CRANK filter removes drift errors by exploiting constraints on the kinematics of the crank arm relative to the bicycle frame. Three 5 min cycling tests were conducted using stereophotogrammetry and two IMMSs; a slow (~80 r/min) and medium (90 r/min) cadence test on a level surface and a fast cadence test (100 r/min) with the bicycle inclined at 20° to the ground. A novel two-segment methodology for collecting ground truth data with an optical motion capture system is presented. We also provide analysis of CRANK filter performance for simulated outdoor dynamics (lateral tilt and roll). The CRANK filter achieved absolute errors (AEs) of 0.9 ± 0.6°, 1.7 ± 1.4°, and 1.8 ± 1.2° for the slow, medium, and fast tests, outperforming a commercial Kalman filter that produced AEs of ~10°. Under simulated outdoor conditions the CRANK filter was only slightly less accurate (AEs ≈ 3°). The CRANK filter is shown to be accurate, drift-free, easy to implement and robust against magnetic disturbances, sensor positioning, bicycle inclination, and bicycle frame dynamics.

The effect of axial rotation of the anterior resection plane in patellofemoral arthroplasty

BACKGROUND Patellofemoral arthroplasty (PFA) has a small but definite place in replacement surgery of the knee, especially in young patients. The main surgical considerations in PFA are the patients anatomy, the type of prosthesis and the surgical technique. The surgical technique and PFA success rely heavily on the anterior resection. In this study we investigate the effect of axial rotation of the anterior resection plane. METHODS We tested the outcome of PFA fit based on resection footprint measurements, axial and coronal groove angles, and lateral trochlear inclination (LTI) angle in a virtual PFA model. The range of anterior resection plane axial rotations was from five degree internal to five degree external with an increment of one degree. RESULTS Axial rotation of anterior resection plane changes the resection footprint dimension, which leads to coronal rotation of the femoral component. External rotation of the resection plane results in valgus rotation of the trochlear groove and decreased LTI after PFA and the opposite was observed for internal rotation. CONCLUSION Our study showed that by changing the axial rotation of the anterior cut, the coronal groove of the prosthesis can be altered to lie more closely with the native groove line without compromising the prosthesis-cartilage transition.

COMPARISON OF COMMERCIAL PATELLOFEMORAL ARTHROPLASTY SYSTEMS ON THE BASIS OF PATELLA KINEMATICS, PERI-PATELLAR SOFT TISSUE TENSION AND PROSTHESIS DESIGN

Patellofemoral arthroplasties are desirable when treating isolated patellofemoral osteoarthritis, due to preservation of the tibiofemoral joint. Since few studies report on new commercial patellofemoral prosthesis biomechanics, a musculoskeletal model enabling analysis of subject-specific knee biomechanics was used to compare four patellofemoral replacement systems (A, B, C, and D) to one another. The prostheses were implanted according to manufacturer guidelines, after which the knee flexed and extended under active muscle loading. An increased patellotrochlear index enabled early patella-trochlear groove engagement. The resurfaced patellae were stable in mediolateral shift and anteroposterior displacement, but only Prosthesis A and D provided a smooth transition between the distal prosthesis border and femoral cartilage. A reduction in the anteroposterior condylar distance displaced the patella posteriorly, resulting in reduced peri-patellar soft tissue tension but an increased patella tendon–quadriceps tendon ratio. The tibial tubercle–trochlear groove distance became pathologic in all replacements. The patella will be stable irrespective of the prosthesis used, but Prosthesis A and D seem to provide a better fit to the trochlear groove anatomy. The increased tibial tubercle–trochlear groove distance emphasizes the importance of extensor alignment in combination with the placement of the prosthesis: an increased Q-angle might lead to excessive lateral wear on the patella button. The extensor mechanism load will increase post-surgery based on the rise in the patella tendon–quadriceps tendon ratio which points to a reduced moment arm. This work provides insight into the dynamic biomechanical function and the design of current commercial patellofemoral replacement systems.

Robust tracking of bicycle crank angles using magneto-inertial sensors, domain constraints and functional frame alignment techniques:

The crank angle is an important outcome in biomechanical analyses of cycling. Wireless inertial and magnetic measurement systems are unobtrusive and have the potential to measure crank angles more advantageously than ergometers, encoders or cameras. However, magnetic field disturbances and large centripetal accelerations during pedaling introduce tracking errors. The aim of this study was to validate two magnetometer-free sensor-to-body frame alignment methods for tracking the bicycle crank angle using wireless inertial and magnetic measurement systems. A passive complementary filter is presented for tracking the crank angle using an inertial and magnetic measurement system mounted on the bicycle frame and another on the crank arm. Sensor-to-body frame alignment is performed for both inertial and magnetic measurement systems using functional calibration techniques that do not require magnetometer measurements. The filter also performs dynamic tracking of the crank arm inertial and magnetic measurement system without magnetometer data by exploiting domain constraints and compensating for centripetal accelerations. The filter was validated at a slow, medium and fast pedaling cadence using stereophotogrammetry. The filter produced absolute errors of 1.3° ± 0.9° or less in all three tests. In contrast, large and variable absolute errors (11.6° ± 7.6°, 14.2° ± 10.7° and 14.0° ± 10.2°, respectively) were found with a standard passive complementary filter using a traditional static pose calibration that relies on magnetometer data. The proposed filter operated with low and consistent errors despite the presence of magnetic interferences, whereas traditional magnetometer-based approaches produced unacceptable results. This study contributes toward the ultimate goal of outdoor cycling analysis using inertial and magnetic measurement system technology by accomplishing magnetometer-free frame alignment and centripetal acceleration compensation when tracking crank angles.

Characterization of a Novel Imaging-Based Metric of Patellofemoral Separation Using Computational Modeling

We introduce patellofemoral separation (PFS) as a novel metric to quantify patella-trochlear proximity as a function of dynamic knee flexion. PFS is quantified in 4D (i.e. 3D+time) using accurate segmentation from pre-operative imaging data acquired in three discrete, quasi-static knee postures, up to the maximum bending limit (i.e. 40° of flexion), within the constraints of a standard computed tomography (CT) or magnetic resonance imaging (MRI) scanner. Additionally, in this study, in order to examine patient-specific patella postures over a full range from 0 to 90° of dynamic knee flexion and extension, we utilize a computational model to simulate dynamic patella kinematics beyond 40° of bending. The computational model was optimized to reproduce patella postures as determined from the imaging data. A method of shape-based interpolation of the acquired 3D components (i.e. bone and cartilage) of the knee was applied in order to recreate a continuous range of motion of the patella and femur during knee bending from 0° to 40° using imaging data and 0° to 90° from simulated data. Next, a regional Hausdorff distance mapping paradigm was applied to compare the separation of the 3D surfaces defined by the patella and femoral cartilage segmentations from the interpolated imaging-based and simulated knee postures, at 1°increments. This separation distance was termed as PFS and examined as a posture-varying color map on the patella cartilage surface. The mean PFS was computed as the mean HD of separation between patella and femoral cartilage, at each posture over the entire studied range of motion. Mean PFS was observed to decrease with increased knee flexion, evidencing increased proximity of the patella and femur and increased risk of contact. In order to automatically quantify signs of patellofemoral instability from pathological knee kinematics reconstructed using medical imaging, the limits of PFS defining the thresholds of pain will require to be determined by benchmarking the metric against patients with normal knee-function. The PFS metric may also find potential application as a biomarker for the identification of high localized patellofemoral pressure by predicting patellofemoral impingement.

Primary Principles in Soft Tissue Balancing

Soft tissue balance and alignment are integral to the success of a total knee arthroplasty (TKA). In 1985 it was already reported that most failures can be attributed to incorrect ligament balance or incorrect alignment [1]. Since 1985, numerous new and improved total knee replacement systems, surgical instruments, surgical methods, and computer-assisted surgery tools have seen the light. Ligament balancing and alignment however still remain the biggest considerations that impact the successful outcome of a total knee arthroplasty.

PATELLOFEMORAL ARTHROPLASTY CHANGES THE TROCHLEAR GROOVE ANGLE

High early failure rates occur in the treatment of isolated symptomatic patellofemoral arthritis with commercially available patellofemoral arthroplasty (PFA) prostheses. We postulate that PFA changes the trochlear groove angle, thereby causing patellar maltracking, catching and pain. We examined the extent of this change in trochlear groove angle by virtually implanting five commercially available patellofemoral prostheses into two 3D reconstructed knees, one with a normal and the other with a dysplastic trochlea. The axial and coronal trochlear groove angles were measured pre- and post PFA for the five different prostheses in both the normal and the dysplastic knee. Post PFA, the trochlear groove angle changed from the original in both the axial and coronal planes for all the prostheses in both the normal and the dysplastic knee. The trochlear groove change is dependent on the design of the specific prosthesis. To avoid excessive changes post PFA, both the wide variation of changes between different generic PFA prostheses, as well as the wide variation in patient femoral anatomy should be considered.

Computational modelling of mobile bearing TKA anterior–posterior dislocation

Anterior–posterior stability in an unconstrained mobile-bearing total knee arthroplasty (TKA) and one with rotational constraints is compared in a computational model based on an ASTM test. Both TKA designs dislocate at loads greater than reported maximum in vivo forces. The posterior drawer forces (mean: 3027 N vs. 1817 N) needed to induce subluxation increase with a greater anterior jump distance (12 mm vs. 7 mm; refers to the vertical height of the anterior or posterior border of the tibial inserts articulating surface). The posterior jump distance for both tested TKA differed by 1.5 mm and had minimal effect on the magnitude of the anterior drawer forces at dislocation in mid-flexion (unconstrained vs. constrained: 445 N vs. 412 N). The unconstrained insert dislocated by means of spin-out whereas in the constrained TKA the femur dislocated from the bearing during posterior drawer and the bearing from the baseplate during anterior drawer. MCL function is an important consideration during ligament balancing since a ± 10% variation in MCL tension affects dislocation forces by ± 20%. The simulation platform provided the means to investigate TKA designs in terms of anterior–posterior stability as a function of knee flexion, collateral ligament function and mechanical morphology.