Introduction and Aims: Single compartmental replacement procedures are increasingly preferred over total knee replacement (TKR) for single compartment osteoarthritis of the knee joint. Theoretically, reduced disruption of the native joint should produce more normal kinematics. This study aimed to describe and compare the sagittal plane kinematics of four different, commonly used devices.

Method: Four groups of patients who had undergone successful single compartment replacement at least two years previously were recruited. Fifteen following Oxford medial UKA, 12 following medial St Georg Sled UKA, five following Oxford lateral UKA, and 12 following Avon PFJ replacement. Patients performed flexion/extension against gravity, and a step-up during video fluoroscopy. The Patellar Tendon Angle (PTA), the angle between the long axis of the tibia and the patella tendon, was obtained as a function of knee flexion. This relationship provides indication of sagittal movement between femur and tibia through range and has been validated as a reliable measure of joint kinematics.

Results: The kinematic profile for each group was compared to that of the profile for 12 normal and 30 TKR (AGC) knees. All three tibiofemoral devices produced knee kinematics similar to the normal knee. The PTA was found to have a linear relationship to flexion angle, decreasing with increasing knee flexion angle. No such linear relationship exists for the TKR joint, which display abnormal kinematics. The PF device also reflected similar trends to that for normal knees except that the PTA was moderately increased throughout the entire range of flexion (three degrees).

Conclusion: In contrast to TKR, all single compartmental knee replacements provided kinematics similar to the normal joint. The kinematic pattern of the PFJ replacement may be of most interest as the observed increase in PTA through range could influence patello-femoral contact forces

Introduction: The Birmingham Hip Resurfacing (BHR) is a metal on metal prosthesis with no published independent clinical studies. It is increasingly used as an alternative to stemmed prostheses in younger patients.

This study presents the 2 year migration results of the BHR femoral component using Roentgen Stereophoto-grammteric Analysis (RSA).

Methods: 26 hips in 24 subjects underwent a BHR, through the postero-lateral approach using CMW3G cement. RSA marker balls were placed in the cortical bone of the femur intra-operatively. The femoral component migration was measured at intervals of 3, 6, 12 and 24 months postoperatively using the Oxford RSA system. The implants were un-modified, geometric algorithms were used to identify the femoral component.

Results: The BHR femoral component showed no significant displacement from its immediate post-operative position. All components of migration showed no significant displacement in any direction, at 1 or 2 years.

Conclusion: The BHR femoral component does not migrate significantly within the first two post-operative years. Its’ migration compares favourably to other designs of femoral component. The BHR is therefore likely to remain stable in the future, as the majority of implant migration occurs within the first two years.

Introduction: This study investigates the influence of surgical approach on the early migration of a cemented Total Hip Replacement (THR), assessed by Roentgen Stereo-photogrammetric Analysis (RSA). Rapid early migration has been correlated with premature implant failure.

Method: 46 patients awaiting THR were allocated into Posterior Approach (PA) and Lateral Approach (LA) groups. RSA was used to assess post-operative stem position and migration at 1 year. Post-operative gait was examined for a subgroup of patients.

Results: There was no difference in initial stem position within the medulla. Significant differences in distal migration were found; (LA 0.78±0.07mm and PA 1.27±0.15mm), p< 0.01. Posterior head migration for the PA group (0.78±0.18mm) was twice that for the LA group (0.46±0.08mm), p< 0.05. The PA group showed greater medial migration of the prosthesis tip (0.62±0.24mm), compared to the LA group (0.11±0.08mm), p< 0.05. During gait, the legs of the PA group were relatively internally rotated when compared to those of the LA group.

Discussion: The PA implants rotate internally and into valgus further than the LA implants. The starting positions of the prostheses were the same with similar cement and prostheses characteristics. We can infer that surgical approach and therefore muscle function have a significant effect upon component migration. During gait, the PA group has relatively internally rotated legs, probably due to short external rotator weakness. Hence, the internally rotating torque on the femoral stem will be larger, explaining the rapid migration in this direction. With the LA approach, the abductors are likely to be damaged. Therefore, the PA joint reaction force will be more laterally directed, explaining the higher coronal plane rotation rates seen in this group.

Conclusion: Profound differences exist in early migration, between the LA and PA. Different muscle function may account for this and probably influences long-term outcome.

Aim: Small amounts of subsidence may be beneficial in stabilising a stem & appears to be a featue of polished tapered stems. Stem rotation (measured as posterior head migration) within the cement mantle, however, is probably a more important mechanism of failure than subsidence. Stems with a wider proximal portion are thought to better resist rotation. The CPS (Endoplus, UK) is such a device; here we compare its’ stability with that of the Exeter (Stryker-Howmedica, UK). Method: 20 patients received the CPS-plus stem & underwent RSA examinations at 3, 6 & 12 months postoperatively. The Exeter 1 year migration data (32 hips) was used as a comparison. Both groups underwent a Hardinge approach & the prosthesis was secured with CMW3G cement. Results: Both stems subsided about 1mm. The CPS showed less medio-lateral & A-P movement of the proximal stem than the Exeter over 1 year, as shown below: 1 Year Mean Migration ± SEM (mm) Discussion: The CPS internally rotates less than the Exeter, as demonstrated by the smaller amount of posterior head migration. It has a lateral flare of the shoulder; making its’ cross-section wider than the Exeter’s, this probably produces a greater resistance to rotation. The CPS also undergoes less medio-lateral proximal stem migration. Its’ lateral shoulder flare is probably responsible for this axial subsidence, as it prevents the shoulder from moving laterally whilst subsiding onto the calcar.

Increasing the width of the proximal section of a polished tapered stem enhances its rotational stability.

The relative motion between a prosthesis, the cement mantle and its’ host bone during weight bearing is not well understood. Using Radiostereophotogrammetric Analysis (RSA), we examined the dynamically inducible micromotion that exists at these interfaces when an increased load is placed through the prosthesis.

Dynamically inducible micromotion was measured in the femoral components of 21 subjects undergoing total hip replacement with polished Exeter stems. Two standing RSA studies were performed, at 3 and 12 months postoperatively. Firstly in double-leg stance, and secondly fully weight bearing through the operated hip.

Subjects had no signs of clinical or radiological signs of loosening at 1 year. Significant micromotion was detected at the prosthesis-cement interface at 3 months.

Similar patterns of micromotion were observed at 12 months. The prosthesis appeared to bend during single-leg stance weight bearing, however this accounted for less than half of the total observed movement.

Conventional RSA studies were conducted at 3 months, 6 months and 1 year to confirm that the implants showed normal migration patterns.

This study demonstrates that movement exists between the prosthesis and bone during cyclical weight bearing. This dynamically inducible micromotion probably occurs at the prosthesis-cement interface. It could account for the wear that is observed on the surface of retrieved secure prostheses. This may be a mechanism by which failure eventually occurs.

The indications for unicompartmental knee arthroplasty (UKA) can be contentious. Concerns exist about implanting a UKA in younger individuals. It has been reported that it is contra-indicated in patients under the age of 60. The suggestion is that younger patients may compromise their outcome after surgery by their increased activity levels. However, the number of publications with data on UKA in young patients is minimal and the age/activity related criteria for UKA remains unknown.

The aim of the study was 1) to confirm that younger UKA patients have higher activity levels than older patients, and 2) to test the hypothesis that younger, more active patients have inferior outcome when compared to older less active patients.

Fifty consecutive patients undergoing UKA who were under 60 years (Group Y) and 50 patients over 60 years (Group O) were recruited. Indications were anteromedial arthritis with full thickness lateral compartment cartilage, a functioning ACL and a correctible varus. All patients underwent Oxford UKA using the minimally invasive technique. This device employs an unconstrained fully congruous meniscal bearing. Outcome was evaluated using Knee Society Scores. Activity level was documented using Tegner Activity Score.

Average age of patients for group Y and O was 55 and 68 years respectively. Minimum follw-up was 2 years. pre-op. knee scores were comparable for each group. (y: 38.8, O: 35.8) and patients in both groups siginificantly improved after surgery. A significant difference in Tegner score was found between the two groups after surgery (y:4.1, O:2.6). More than 50% of younger patients regularly participated in high demand activities like skiing, tennis and manual labour. No statistically significant differences in any other post op. knee scores or complication rate were found despite adequate study power.

Younger patients have increased activity levels after UKA when compared to an older age group. There is no evidence that the outcome of UKA in younger more active patients will be inferior to those who are older and more sedentary.

The Birmingham reSurfacing Arthroplasty (BSA) is a metal on metal prosthesis with no published independent clinical studies. Despite this, it is increasing in popularity, especially as an alternative to stemmed prostheses in younger patients. This study presents the 1year migration results of the BSA femoral component using Roentgen Stereophotogrammteric Analysis (RSA).

Twenty six subjects underwent a BSA, through the postero-lateral approach using CMW3G cement, with RSA marker balls placed intra-operatively. The femoral component migration was measured at intervals of 3, 6 and 12 months using the Oxford RSA system. Geometric algorhythms were used to identify the femoral component. The data was examined for distribution prior to analysis. All statistical analysis was performed using the t-test.

The data was normally distributed. The 1 year migration results of the BSA femoral component are displayed below.

All cemented implants migrate in vivo. The majority of cemented stemmed implant migration occurs within the first post-operative year. High rates of migration within the first post-operative year correlate with premature component failure in some instances. The BSA is a fundamentally different design to most cemented prostheses, despite this we know that very large migrations, those in excess of 2mm/year in any direction are generally regarded as poor indicators of long term outcome. These results suggest that the BSA femoral component is an inherently stable device as it does not migrate significantly within the first post-operative year. Only long-term independent clinical studies and continued RSA follow-up will enable a comprehensive evaluation of the device.

This study examined the effect of surgical approach on the 1year migration of the Exeter stem, using Roentgen Stereophotogrammetric Analysis (RSA). There is evidence that implants with increased early migration, particularly those moving into internal rotation, are likely to have a higher failure rate.

A total of 46 patients awaiting THR were allocated into 2 groups. Both groups underwent an Exeter Total Hip replacement, one through the Posterior Approach (PA), and the other through the Hardinge Approach (HA). RSA was used to assess post-operative stem position and migration at 1 year. Post-operative gait analysis and clinical evaluation was used to assess a subgroup of patients.

There was no difference in the initial stem position of each group within the femoral canal. Significant differences in migration were found for migration of the head distally and posteriorly and for the tip medially. During gait, the legs of the PA group were relatively internally rotated when compared to those of the HA group. Relative to the supporting bone, the PA implants internally rotate and also rotate into valgus further than the HA implants. The starting position of the prostheses was the same and the cement and prosthesis characteristics were similar. From this, we can infer that surgical approach and therefore muscle function, have a large influence on component migration. During gait, the PA group had internally rotated legs compared to the HA group. This is probably due to short external rotator weakness in the PA group. As a result the internally rotating component of the posteriorly directed joint reaction force on the femoral stem will be larger. This probably explains the rapid internal rotation seen in the implants of the PA group. With the HA group, because of abductor damage there is likely to be a smaller component of the joint reaction force acting on the stem. This would explain the smaller rates of observed coronal plain rotation (rotation into valgus).

Profound differences exist in early migration, between the HA and PA. Different muscle function may account for this and probably influences long-term outcome.

Introduction: The indications for unicompartmental knee arthroplasty (UKA) can be contentious. Concerns exist about implanting a UKA in younger individuals and it has been reported that the procedure is contra-indicated in patients under the age of 60. The suggestion is that younger patients may compromise their outcome after surgery by their increased activity levels. However, the number of publications with data on UKA in young patients is minimal and the age/activity related criteria for UKA remains unknown.

Aim: The aim is twofold: 1) to confirm that younger UKA patients have higher activity levels than older patients, and 2) to test the hypothesis that younger, more active patients have inferior outcome when compared to older less active patients.

Materials and methods: Fifty consecutive patients undergoing UKA who were under 60 years old at the time of surgery (Group Y) and 50 patients over 60 years (Group O) were recruited. Indications were anteromedial arthritis with full thickness lateral compartment cartilage, a functioning ACL and a correctible varus deformity. All patients underwent Oxford UKA using the minimally invasive technique. This device employs an unconstrained fully congruous meniscal bearing. Outcome was evaluated using the American Knee Society Score (AKSS). Activity level was documented using the established Tegner Activity Score.

Results: The average age of patients for group Y and O was 55 years and 68 years respectively. Minimum follow up for both groups was 2 years. Pre-operative AKSS scores were comparable for each group (Group Y = 38.8, Group O = 35.8) and patients in both groups significantly improved after surgery. A significant difference in Tegner score was found between groups after surgery (Group Y = 3.9, Group O = 2.6). It was found that 40% of younger patients regularly participate in high demand activities like skiing, tennis, hard manual labour and swimming. No statistically significant difference in any other post operative knee scores or complication rate were found despite adequate study power.

Conclusions: Younger patients have increased activity levels after UKA when compared to an older age group. There is no evidence that the outcome of UKA in younger more active patients will be inferior to those who are older and more sedentary.