Pieter J. Erasmus

Management of overtight medial patellofemoral ligament reconstruction

The clinical presentation of an overtight medial patellofemoral ligament (MPFL) reconstruction can differ depending on whether it is too tight in extension (extensor lag) or too tight in flexion (anterior knee pain and loss of flexion). We report one clinical case of each presentation. Both cases were treated with a percutaneous release of the graft. After the release, both patients regained a full active range of motion without residual symptoms. These complications demonstrate that the adjustment of the graft tensioning as well as its femoral position are critical steps in MPFL reconstruction. This procedure requires training and experience in order to avoid early complications related to malposition or inappropriate tensioning of the graft. A surgical management for these overtight reconstructions is recommended, as it will restore function and range of motion, and prevent late patellofemoral degeneration.

Recurrent patellar dislocation after medial patellofemoral ligament reconstruction

We report on three cases of recurrent lateral patellar dislocation following a medial patellofemoral ligament (MPFL) reconstruction for patellar instability. In all three cases, an isolated MPFL reconstruction was performed with a double autogenous gracilis graft. The patellar fixation was done through bone tunnels. All three patients presented with a definite moderate to severe traumatic episode resulting in a recurrent patella dislocation and a transverse avulsion fracture at the medial rim of the patella. All three were treated by an open reduction and internal fixation with good results. No complication or recurrent dislocations occurred. We suggest that this complication is caused by the original underlying pathology such as dysplastic trochlea, abnormal TT–TG, patella alta and hyperlaxity, resulting a greater reliance upon the reconstructed MPFL for patellar stability. When subjected to a severe stress, the graft, which is stronger and stiffer than the original MPFL, will cause a fracture through the medial edge of the patella. This weak area results from the previous drill holes, which act as stress risers.

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.

Contact stresses in a patient-specific unicompartmental knee replacement

BACKGROUND Unicompartmental knee replacement has gained popularity in recent times, showing improved success rates. The main reasons for the failure of unicompartmental knee replacement are the wear of the polyethylene bearing, aseptic loosening and wear in the opposite compartment. The contact stresses involved are significant contributing factors to these causes of failure. METHODS In this study, a patient-specific unicompartmental knee replacement is proposed using a methodology based on neural network modeling of a database of healthy knee geometries. This custom implant was then compared to two conventional implant designs in terms of contact stress in a validated finite element model. FINDINGS The custom implant experienced lower contact stresses at the tibio-femoral joint compared to a fixed-bearing design and also displayed more uniform stress distribution at the bone-implant interface than any of the other implant designs. INTERPRETATION Custom unicompartmental knee replacements therefore have the potential of providing good contact stress distribution, preserve bone stock and could be more anatomically accurate.

Anatomical Study of the Radius and Center of Curvature of the Distal Femoral Condyle

In this anatomical study, the anteroposterior curvature of the surface of 16 cadaveric distal femurs was examined in terms of radii and center point. Those two parameters attract high interest due to their significance for total knee arthroplasty. Basically, two different conclusions have been drawn in foregoing studies: (1) The curvature shows a constant radius and (2) the curvature shows a variable radius. The investigations were based on a new method combining three-dimensional laser-scanning and planar geometrical analyses. This method is aimed at providing high accuracy and high local resolution. The high-precision laser scanning enables the exact reproduction of the distal femurs-including their cartilage tissue-as a three-dimensional computer model. The surface curvature was investigated on intersection planes that were oriented perpendicularly to the surgical epicondylar line. Three planes were placed at the central part of each condyle. The intersection of either plane with the femur model was approximated with the help of a b-spline, yielding three b-splines on each condyle. The radii and center points of the circles, approximating the local curvature of the b-splines, were then evaluated. The results from all three b-splines were averaged in order to increase the reliability of the method. The results show the variation in the surface curvatures of the investigated samples of condyles. These variations are expressed in the pattern of the center points and the radii of the curvatures. The standard deviations of the radii for a 90 deg arc on the posterior condyle range from 0.6 mm up to 5.1 mm, with an average of 2.4 mm laterally and 2.2 mm medially. No correlation was found between the curvature of the lateral and medial condyles. Within the range of the investigated 16 samples, the conclusion can be drawn that the condyle surface curvature is not constant and different for all specimens when viewed along the surgical epicondylar axis. For the portion of the condylar surface that articulates with the tibia during knee flexion-extension, the determined center points approximate the location of the centers of rotation. The results suggest that the concept of a fixed flexion-extension axis is not applicable for every specimen.

Classification of gender and race in the distal femur using self organising maps.

In this study gender and race differences in distal femoral morphology were investigated. Reliable anatomic knee measurements were obtained for 60 knees via MRI and direct scanning of cadaver specimens. The MRI data comprised of 20 White males and 22 White females while the cadaver specimens comprised of 18 Black males. Possible differences were investigated using a type of artificial neural network to classify the data, namely the self-organising map (SOM). The SOM suggested that clear differences are present between genders when absolute measurements are used. Male knees tended to be larger over all the measurements considered. However, when data were normalised for size, the clear differences were diminished and definite clusters were difficult to define. Black male knees tended to have larger condyle radius to anterior-posterior length ratios compared to White males. White male knees tended to be wider than White female knees. It is however suggested than when corrected for size, there exists a large variation among individual knees regardless of gender or race. It is argued that with the large variation in populations it can become advantageous not to think about gender-specific or race-specific knee replacement designs, but rather patient-specific.

Method for selection of femoral component in total knee arthroplasty (tka).

A method is proposed enabling a surgeon to preoperatively determine the preeminent type and size of prosthesis, from those available, to be used in a particular patient undergoing knee replacement surgery. Parameters of healthy knee geometry were estimated by employing an unsupervised neural network. These estimated parameters were then applied in a χ2 goodness of fit (GoF) test to determine which femoral prosthesis type and size delivers the most appropriate fit. This approach was used to determine the most suitable match of three implants for 34 different cases. Implant C performed the best and was the optimal fit in 59% of the cases, Implant A was the best fit in 38% of the cases and Implant B the best fit in 3% of the cases. This method shows promise in aiding a surgeon to select the optimal prosthesis type and size from an array of different conventional total knee replacements.

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.

Application of an artificial neural network for the quantitative classification of trochlear dysplasia

A method that provides the ability to accurately classify trochlea dysplasia would be a valuable tool in treating patients with unstable patellas and anterior knee pain. In this study, a reference frame and a standardised femoral parameter measurement method for three-dimensional models were established to measure key femoral parameters. An artificial neural network was then trained with these parameters and the matching output from qualitative classifications by three orthopaedic surgeons for each knee. The neural network was then evaluated to test its ability for quantitative classification of normal and dysplastic knees. The maximum agreement between the qualitative and quantitative classification methods was found to be 80.6%, whereas agreement between the surgeons was 69.44%.This was achieved for a neural network with 8 input parameters and 19 hidden-layer neurons. The study shows that there is merit in making use of a trained artificial neural network as an additional tool for classification of trochlear dysplasia.

Development and testing of patient-specific knee replacements

This study presents a design methodology for designing and manufacturing patient-specific unicompartmental knee replacements. The design methodology uses mathematical modeling and an artificial neural network to predict the original and healthy articulating surfaces of a patients knee. The models are combined with medical images from the patient to create a knee prosthesis that is patient-specific. These patient-specific implants are then compared to conventional implants with respect to contact stresses and kinematics. The patient-specific implant experienced lower contact stresses at the tibiofemoral joint compared to a fixed-bearing design. Both the UKRs showed similar kinematic patterns to the normal knee using two different test rigs. The patient-specific UKR showed good results and with the other benefits it shows potential to dramatically improve clinical outcomes of knee replacement surgery.