Introduction: The results of the mobile bearing Oxford partial knee replacement (PKR) in the lateral compartment have been disappointing with a five year survival of 82%. Bearing dislocation is a particular concern, and to address this issue a new domed implant was introduced with a modified surgical technique. The aim of this study was to compare the risk of dislocation between a domed and flat lateral PKR.

Methods: Separate kinematic models were generated for the domed and flat bearings. The femoral component, tibial tray and bearing were aligned in a neutral position; the flat bearing was positioned centrally on the tibial tray and 2 mm from the side wall; the domed bearing was placed concentrically on the domed tibial tray. Dislocation in the Posterior (A-P), Lateral (M-L) and Medial against the tray wall (L-M-wall) were investigated. For each dislocation the tibial tray was restrained in all degrees of freedom (DOF) and the femoral component was restrained in five DOF; A-P and M-L displacements; A-P, M-L and Superior-Inferior (S-I) rotations. The bearing was restrained from rotating about the S-I axis for each dislocation. For the L-M-wall dislocation the underside of the bearing was held in contact with the tibial tray wall such that the lowest S-I displacement of the femoral component was achieved. The least amount of distraction required for bearing dislocation to occur was calculated for the seven bearing sizes available. The effect of medial-lateral positioning of the femur on dislocation was investigated.

Results: The minimum femur distraction to cause A-P flat and domed bearing dislocation ranged from 4.68mm to 3.91mm and 6.29mm to 5.59mm respectively as the bearing thickness increased from 3.5mm to 9.5mm. The minimum femur distraction to cause L-M-wall flat and domed bearing dislocation ranged from 3.42mm to 4.16mm and 4.55mm to 5.44mm respectively as the bearing thickness increased from 3.5 mm to 9.5 mm. The femur distraction required for L-M-wall bearing dislocation increased from 4.55mm to 6.3mm with a 2 mm medial movement of the femoral component. A 4 mm lateral movement of the femoral component decreased the distraction from 4.55mm to 2.35mm.

Discussion: A domed bearing can lead to an increased femoral distraction of between 25% and 37%, significantly reducing the likelihood of dislocation. This may be significant during everyday activities and demonstrates that the new domed design should reduce the incidence of bearing dislocation by increasing the amount of entrapment; our current series of 200 patients has no dislocations. Increasing the thickness of the bearing has a small effect on the distraction required to allow bearing dislocation. The medial-lateral placement of the femoral component has a pronounced effect on the femoral distraction required for bearing dislocation over the tray wall; medial placement of the femoral component is advisable.

A linkage-based mathematical model was used to design a ligament-compatible prosthesis to keep certain ligament fibres isometric during passive motion. The sagittal plane talar component radius is about 50% longer than that of the normal talus, the tibial component is spherically convex. A fully conforming meniscal bearing is interposed between them. Experiments in cadaver specimens confirmed the mathematical prediction that the bearing moves forwards on both metal components during dorsi-flexion and backwards during plantar flexion.

Between July 2003 and July 2008, the prosthesis was implanted into 250 patients at nine hospitals in Northern Italy. By November 2007, 158 in 156 patients were seen at least six months post-operatively. Mean age was 60.5 years. The diagnosis was post-traumatic osteoarthritis in 127, primary osteoarthritis in 17, rheumatoid arthritis in 10.

The mean follow-up was 32.5 months. The pre-operative AOFAS score of 36.2 rose to 75.9, 79.3, 77.9, and 79.0 respectively at 12, 24, 36, 48 months. Dorsi-flexion increased from 0.1° to 9.7°, plantarflexion from 15.1° to 24.6°. In 30 patients at one hospital, the range of postoperative motion, 14° – 53°, was significantly correlated to the range of bearing movement on the tibial component, 2mm–11mm, measured radiologically, (r2 = 0.37, p < 0.0005).

By December 2007, 2 revision operations had been performed at 24 months, one for unexplained pain not relieved by a successful arthrodesis, one in a patient with Charcot-Marie-Tooth disease. There were no device-related revisions (loosening, fracture, dislocation). The Kaplan-Meier survival rate (component-removal as end-point) at 4 years was 96% (Confidence interval 90–100%).

Early clinical results have demonstrated safety and efficacy. The survival rate at four years compares well with multi-centre 5-year rates published by the Swedish (531 cases, survival 78%), Norwegian (257, 89%) and New Zealand (202, 86%) registries.

The design philosophy of polished tapered total hip replacements (THR), such as the Exeter, intends for them to migrate distally within the cement mantle. As well as migration, dynamically induced micromotion (DIMM) occurs as a result of functional activity between the implant and the cement. The aim of the current study was to develop and validate a finite element (FE) model of the Exeter/cement/bone system which can be used to predict DIMM and investigate the stresses induced in the cement mantle during functional activity.

In the context of the current study, DIMM is defined as the displacement of the implant component relative to the bone when moving from double leg stance to single leg stance on the operated limb. Using Roentgen Stereo-photogrammetric Analysis (RSA), DIMM was measured in 21 patients implanted with Exeter stems 3 months post-operatively. A previous study, using a reduced FE model of the Exeter stem and the surrounding cement mantle focused on the solution of the contact problem at the stem-cement interface. It was demonstrated that sliding contact combined with Coulomb friction and an appropriate parameter setting could be used to predict DIMM of a polished tapered stem. For the purposes of the current study, the previous simple model was incorporated into the FE model of the Muscle Standardised Femur and validated against the RSA measurements for DIMM. For the current extended model, loading included muscle forces representing all active muscles acting on the femur. The effect of initial cement stresses and interdigitation was also considered.

The Exeter stem demonstrated significant DIMM (p< 0.017). The FE model, accounting for sliding contact at the cement–implant interface was able to predict similar distal migration of the head and the tip. The results of both the calculations and the measurements showed that the femoral head moves medially, distally and posteriorly relative to the bone. In the cement mantle, maximum principal stresses were oriented circumferentially, minimum principal stresses were oriented radially. When the taper got engaged, submicroscopic movements which did not recover following unloading still took place and accumulated.

The results of the present study showed that it is possible to measure DIMM in the Exeter stem and combine this with FE modelling of the contact mechanism. Future studies will include various activities, such as walking or stair climbing. Based on accumulated submicroscopic movements, short-, mid- or long-term migration patterns will be predicted.

Aims: Prior research has demonstrated that currently available total ankle implants fail to restore physiologic joint mobility. Most of the modern mobile-bearing designs that feature a flat tibial component and a talar component with anatomic curvature in the sagittal plane function non physiologically with the natural ligament apparatus. The aims of this investigation were a) to elucidate the natural relationship between ligaments and articular surfaces at the intact human ankle joint and b) to develop a new design of total ankle replacement able to replicate this relationship between the retained ligaments and the implanted prosthetic components.

Methods: Motion during passive flexion was analyzed in ten skeleto-ligamentous lower leg preparations including tibia, fibula, talus, calcaneus and intact ligaments. Geometry of ligament fiber arrangement and articular surface shapes was obtained with a 3D digitizer (FARO Technologies, Inc.). A sagittal four-bar linkage model was formulated as formed by the tibia/fibula and talus/ calcaneus rigid segments and by the calcaneofibular and tibiocalcaneal ligaments. To test the ability of possible new prostheses to reproduce the compatible mutual function between the articulating surfaces and the ligaments retained, non-conforming two-component and fully-conforming three-component designs were analyzed. A new total ankle replacement has been designed, prototypes manufactured and implanted in seven skeleto-ligamentous lower leg preparations, and motion was observed. A corresponding new prosthesis has been produced (Finsbury, UK), and implanted in four patients.

Results: The articular surfaces and the ligaments alone prescribed joint motion into a preferred single path of multiaxial rotation (one degree of unresisted freedom). Fibers within the calcaneofibular and tibiocalcaneal ligaments remained most isometric throughout the passive range. The four-bar linkage model well predicted the sagittal plane kinematics observed in corresponding experiments. A ligament-compatible, convex-tibia, fully-congruent, three-component prosthesis design showed the best features: complete congruence over the entire range of flexion together with an acceptable degree of entrapment of the meniscal bearing. Restoration of natural joint kinematics and ligament recruitment was observed in all replaced ankles.

Conclusions: The overall investigation is demonstrating that a profound knowledge of the changing geometry of the joint passive structures throughout the range of passive flexion (mobility) is mandatory for a successful design of joint replacements.

Only recently has the mobility of the ankle joint been elucidated. Sliding/rolling of the articular surfaces and slackening/tightening of the ligaments have been explained in terms of a mechanism guided by the isometric rotation of fibres within the calcaneofibular and tibiocalcaneal ligaments. The purpose of this investigation was to design a novel ankle prosthesis able to reproduce this physiological mobility.

A four-bar linkage computer-based model was used to calculate the shapes of talar components compatible with concave, flat and convex tibial components and appropriate fully congruous meniscal bearings. Three-component designs were analysed, and full congruence of the articular surfaces, appropriate entrapment of the meniscal bearing and isometry of the two ligaments were required.

A convex tibial component with 5 cm arc radius gave a 2 mm entrapment together with a 9.8 mm amount of tibial bone cut, while maintaining ligament elongation within 0.03 % of the original length. The physiological patterns of joint motion and ligament tensioning were replicated. The talar component slid backwards while rolling forwards during dorsiflexion. These movements were accommodated by the forward displacement of the meniscal bearing on the tibial surface under the control of the ligaments. The complementary surfaces provide complete congruence over the entire range of flexion, such that a large contact area is achieved in all positions.

To restore the physiological mobility at the ankle joint, not only should the components be designed to be compatible with original ligament pattern of tensioning, but also these should be mounted in the appropriate position. A suitable surgical technique was devised and relevant instrumentation was manufactured. Five below-knee amputated specimens replaced with corresponding prototype components showed good agreement with the model predictions.

Current three-component designs using a flat tibial component and physiological talar shapes cannot be compatible with physiological ligament function.

When the Oxford unicompartmental meniscal bearing arthroplasty (UCA) is used in the lateral compartment of the knee 10% of the bearings dislocate. An in-vitro cadaveric study was performed to investigate if the anatomy and joint geometry of the lateral compartment was a contributory factor in bearing dislocation. More specifically, the study investigated if the soft tissue tension of the lateral compartment, as determined by the length of the lateral collateral ligament (LCL), was related to bearing dislocation. A change in length of greater than 2 mm is sufficient to allow the bearing to dislocate.

The Vicon Motion Analysis System (Oxford Metrics, Oxford, UK) was used to assess length changes in the LCL of seven cadaveric knees. Measurements were made of the LCL length through knee flexion and of the change in LCL length when a varus force was applied at a fixed flexion angle. Measurements were made in the normal knee and with the knee implanted with the Oxford prosthesis.

In the intact knee the mean LCL change was 5.5mm (8%) over the flexion range. After implantation with the Oxford UCA the mean change in length was only 1 mm (1%). There was a significant difference in the LCL length at 90° (p=0.03) and 135° (p=0.01) of knee flexion compared to the intact knee. When a varus force was applied the LCL length change of the intact knee (5.4 mm) was significantly different (p=0.02) to that of the knee with the prosthesis implanted (2.7 mm)

This study used a new method to dynamically measure LCL length. It found that after implantation of the Oxford lateral UCA the LCL remains isometric over the flexion range and does not slacken in flexion as it in the normal knee. This would suggest that the soft tissue tension was adequate to contain the bearing within the joint.

However, when a varus force was applied the LCL did not sufficiently resist a displacing force producing an LCL length change greater than 2 mm.

The evidence provided by this study is contradictory. The “lack of change in LCL length through flexion” suggests that the ligament remains tight through range and is unlikely to allow dislocation. However, the amount of distraction possible when an adducting moment is applied is sufficient to allow bearing dislocation. The length tension properties of the lateral structures are therefore implicated in the mechanism of dislocation.

Purpose: As part of the step-wise introduction of a meniscal-bearing total knee replacement (Oxford TMK) we needed to know, before proceeding to longer term studies, whether its early clinical results were at least as good as those of an established fixed bearing device (AGC).

Material and Methods: With ethical approval, patients requiring bilateral knee replacement for osteoarthritis consented to have the operations under one anaesthetic using one of each prosthesis; to accept random choice of knee; and to remain ignorant which side was which. American Knee Society Scores, Oxford Knee Scores, ROM and pain scores were to be recorded preoperatively and at one year. By January 2001, 40 patients had reached one year and data is available for 36.

Results: Preoperatively there was no difference between the two knees. One patient died in the peri-operative period.

Results at one year (TMK first): AKSS(Knee) 91.6 / 84.1 (p=0.003), OKS 39.8 / 37.6 (p=0.006), ROM 104 / 104 (p=0.364), Pain (AKSS) 47.3 / 41.7 (p=0.01), Pain (OKS) 3.5 / 2.9 (p=0.006).

Conclusion: The TMK performed as well as the AGC. Its AKSS, OKS and pain scores were significantly better. We believe this controlled, blinded trial is the first to have compared the function of a new knee prosthesis with a standard implant before marketing; and the first to have demonstrated a significant clinical advantage for a meniscal-bearing over a fixed bearing TKR. The comparison of bilateral implants in the same patient can reveal significant differences while putting at risk many fewer subjects than would be needed for a classical twocohort RCT.

We determined the outcome of 56 ‘Oxford’ unicompartmental replacements performed for anteromedial osteoarthritis of the knee between 1982 and 1987. Of these, 24 were in patients who had died without revision, one was lost to follow-up and two had been revised. Of the remaining 29 knees, 26 were examined clinically and radiologically, two were only examined clinically and one patient was contacted by telephone. The mean age of the patients was 80.3 years.

At a mean follow-up of 11.4 years (10 to 14) the measurements of the knee score, range of movement and degree of deformity were not significantly different from those made one to two years after operation, except that the range of flexion had improved. Comparison of fluoroscopically-controlled radiographs at a similar interval of time showed no change in the appearance of the lateral compartments. The retained articular cartilage continued to function for ten or more years which suggests that anteromedial osteoarthritis may be considered as a focal disorder of the knee. This justifies continued efforts to develop methods of treatment which preserve intact joint structures.