Due to shorter hospital stays and faster patient rehabilitation Unicompartmental Knee Replacements (UKR) are now considered more cost effective than Total Knee Joint Replacements (TKJR). Obesity however, has long been thought of as a relative contraindication to UKR due to an unproven theoretical concern of early loosening. This study is a retrospective review of patient reported outcome scores and revision rates of all UKR with recorded BMI performed by the Canterbury District Health Board (CDHB) from January 2011 and September 2021. Patient reported outcome scores were taken preoperatively, at 6 months, 1 year, 5 years and 10 years post operatively. These included WOMAC, Oxford, HAAS, UCLA, WHOQOL, normality, pain and patient satisfaction. 873 patients had functional scores recorded at 5 years and 164 patients had scores recorded at 10 years. Further sub-group analysis was performed based on patient BMI of <25, 25–30, 30–35 and >35. Revision data was available for 2377 UKRs performed in Christchurch during this period. Both obese (BMI >30) and non-obese (BMI <30) patients had significantly improved post-operative scores compared to preoperative. Pre-operatively obese patients had significantly lower functional scores except for pain and UCLA. All functional scores were lower in obese patients at 5 years but this did not meet minimum clinical difference. At 10 years, there was significantly lower HAAS, satisfaction and WOMAC scores for obese patients but no difference in Oxford, normality, WHOQOL, UCLA and pain scores. There was no significant difference in the improvement from pre-operative scores between obese and non-obese patients. All cause revision rate for obese patients at 10 years was 0.69 per 100 observed component years compared to 0.76 in non-obese. This was not statistically significant. Our study proves that UKR is an excellent option in obese patients with post-operative improvement in functional scores and 10 year survivorship equivalent to non-obese patients

Abstract. Introduction. Patient selection is key to the success of medial unicondylar knee arthroplasty (UKA). Progression of arthritis is the most common indication for revision. Various methods of assessing the lateral compartment have been used including stress radiography, radioisotope bone scanning, MRI, and visualisation at the time of surgery. Arthroscopy is another means of assessing the integrity of the lateral compartment. Methods. We used per-operative arthroscopy as a means to confirm suitability for UKA in a consecutive series of 279 Oxford medial UKA. This study reports the long-term results of a previously published cohort of knees. Our series of UKA with per-operative arthroscopy (Group 1) was compared to all Oxford UKA (Group 2) and all UKA in the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) (Group 3). Results. The 14-year cumulative percentage revision (CPR) was 18.5% (95% CI 12.7, 26.4) for group 1, 19.7% (95% CI 18.8, 20.6) for group 2, and 19.2% (95% CI 18.5, 19.8) for group 3. There was no statistically significant difference in the (CPR) for the entire period when group 1 was compared to groups 2 or 3. Progression of arthritis was the indication for revision in similar proportions for the three groups (Group 1: 32.3%, Group 2: 35.7% and Group 3: 33.5%). Following per-operative arthroscopy 21.6% (77/356) of knees underwent a change of surgical plan from UKA to TKA. Conclusion. Per-operative arthroscopy may improve medium-term medial unicompartmental knee survivorship but does not improve long-term survivorship nor reduce revision due to progression of arthritis

A review of current literature describes varying 10-year survival rates for the Oxford Unicompartmental Knee Replacement (Biomet Orthopedics Inc, Warsaw, Ind). Application of rigorous indications and meticulous surgical technique are two factors considered to reduce revision rates. A retrospective case-note review was conducted for 96 patients (128 knees) aged 42–89 (mean 57) who had an Oxford unicompartmental knee replacement for medial compartment osteoarthritis between January 2000 and January 2011. All procedures were performed, or directly supervised, by one 5 surgeons. The aim of the study was to ascertain the rate of revision to bicompartmental knee replacement and any associated contributory factors. Of the 128 unicompartmental knees, 10.9% were revised to either mobile- or fixed-bearing total knee replacements due to septic (0.5%) and aseptic (1.5%) loosening, patello-femoral pain (3.9%), periprosthetic fracture (0.8%) and bearing dislocation (3.1%). Of those knees requiring revision, mean patient age was 73 years, 50% had wound complications and 42% were performed by senior trainees. All patients had intact ACL and medial osteoarthritis. Mean time to revision was 2.7 years. In conclusion, revision of the unicompartmental knee was related to patient age > 65 years and early post-operative complications; grade of operating surgeon had little apparent effect

Introduction: Unicomparmental knee replacements have a long clinical history of success as well as failure. Recently, in Australia some 40% of knee surgery performed consists of unicompartmental knees for the treatment of medial compartment OA. This increased use of unicompartmental knees is in part due to advances in surgical technique through a minimally invasive approach. Loading conditions at the tibia-implant interface will play an important role in the stress/strain distributions at the proximal tibia. The use of an all PE tibial insert versus a metal backed component may provide a different strain disribution to the proximal tibia. This study examined the influence of metal backed and polyethylene tibial components in unicompartmental knee replacements with and without cement fixation on the initial strain distributions under various loading conditions. Materials and Methods: Three cadaveric tibias (mean age 47 years old) were cleaned of all soft tissue and strain gauged. Rosette strain gauges (TML Ltd., Tokyo, Japan) were placed at 2 levels on the tibial cortex. The intact tibia were embedded in a low melting point alloy at a standard height and tested using an MTS 858 Bionix testing machine (MTS Systems, Min., MI). The tibia were tested in nuetral, varus and valgus positions at zero and sixty degrees of flexion. A 1500N was applied for 15 seconds and the strains measured. A K-Scan sensor (Tekscan, Boston, MA) was used to confirm the varus and valgus loading positions and to obtain a contact footprint and pressure for the intact and reconstructed tibias under the loading conditions (Fig. 1). Following intact testing, the tibias were templated and reconstructed by a surgeon familiar with the technigue. The implants were investigated with and without cement fixation and compared to their respective all polyethylene component if it was available using the same loading regime as the intact tibias. Principal strains were calculated. Results: Tibial cortical strain distributions were significantly different at the proximal and distal sites under the loading conditions examined. The strain distribution for metal backed components was greater than the all PE design. Increasing flexion angle shifted the peak strains posteriorly. Metal backing and all PE tibial inserts presented different strain distributions on the medial side under nuetral and varus loading. Lateral compartment strains did not differ between designs, were higher proximal and decreased dramatically at the distal gauges. Cementless fixation tended to overload compared to the intact condition. Figure 2 presents the strain distribution for a typical metal backed and all poly unicompartmental knee in the nuetral position. Discussion: Metal backed unicompartmental components overloaded the proximal cortex of the tibia. All polyethylene tibial inserts did not overload the proximal cortex and had similar strain distribution to the intact tibia. Cemented fixation allows the transfer of load to the distal tibial cortex via the proximal cortex and subchondral bone, provided that the bone cement has inter-digitised the subchondral bone

The purpose of this study is to assess the long term results of combined ACL reconstruction and unicompartmental knee replacements (UKR). These patients have been selected for this combined operation due to their combination of instability symptoms from an absent ACL and unicompartmental arthritis.

Retrospective review of 44 combined UKR and ACL reconstruction by a single surgeon. Surgeries included both medial and lateral UKR combined with either revision ACL reconstruction or primary ACL reconstruction. Patient reported outcomes were obtained preoperatively, at one year, 5 years and 10 years. Revision rate was followed up over 13 years for a mean of 7.4 years post-surgery.

The average Oxford score at one year was 43 with an average increase from pre-operation to 1 year post operation of 15. For the 7 patients with 10 year follow up average oxford score was 42 at 1 year, 43 at 5 years and 45 at 10 years.

There were 5 reoperations. 2 for revision to total knee arthroplasty and 1 for an exchange of bearing due to wear. The other 2 were the addition of another UKR. For those requiring reoperation the average time was 8 years.

Younger more active patients presenting with ACL deficiency causing instability and unicompartmental arthritis are a difficult group to manage. Combining UKR and ACL reconstruction has scant evidence in regard to long term follow up but is a viable option for this select group. This paper has one of the largest cohorts with a reasonable follow up averaging 7.4 years and a revision rate of 11 percent.

Combined unilateral knee replacements and ACL reconstruction can be a successful operation for patients with ACL rupture causing instability and unicompartmental arthritis.

Two computer assisted techniques (CT and a fluoro-guide based system) were used to insert the femoral component of the Oxford Unicompartmental Knee arthroplasty. The accuracy and variability of component positioning were compared. Clinical data was collected pre-operatively and is being collected post-operatively. Standing AP and lateral knee X-rays as well as skyline X-rays were collected pre-operatively and post-operative full length AP and lateral femur X-rays were completed in order to measure alignment of the femoral component. Results are showing accurate insertions of the Oxford knee femoral component using both systems. To review two computer-assisted techniques for inserting Oxford Unicompartmental Knee arthroplasties. CT based and fluro based techniques were compared with regards to accuracy and variability of component positioning. Currently we are able to use either a CT based system or a fluro based system to accurately insert the femoral component of the Oxford Unicompartmental Knee arthroplasty. Computer assist techniques are allowing us to perform minimally invasive arthroplasty procedures with great accuracy. Patients were all seen in a pre-admission clinic where pre-operative clinical survey data were collected. All patients had standing AP and lateral knee X-rays as well as skyline X-rays pre-operatively. Post-perative full length AP and lateral femur X-rays were completed in order to measure alignment of the femoral component. Patients are being followed post-operatively with SF-36, WOMAC, Knee Society Scores, and X-rays. Patients being operated on with the CT based system had pre operative CT scans. Intra-operatively a DRB was fixed to the patient’s femur and the chosen computer assisted technique was used to direct the rotation of the tibial cut as well as the alignment of the femoral cutting jig. To date we have completed seventeen computer assisted Oxford Unicompartmental Knee Arthroplasties. The average error in the AP plane using CT based system was 3.2 degrees and 2.1 degrees for the lateral plane. The average error in the AP plane using the fluro-based system was 2.2 degrees and 1.3 degree for the lateral plane. Funding: NSERC, IRIS, ORDCF

Two computer assisted techniques (CT and a fluoro-guide based system) were used to insert the femoral component of the Oxford Unicompartmental Knee arthroplasty. The accuracy and variability of component positioning were compared. Clinical data was collected pre-operatively and is being collected post-operatively. Standing AP and lateral knee X-rays as well as skyline X-rays were collected pre-operatively and post-operative full length AP and lateral femur X-rays were completed in order to measure alignment of the femoral component. Results are showing accurate insertions of the Oxford knee femoral component using both systems. To review two computer-assisted techniques for inserting Oxford Unicompartmental Knee arthroplasties. CT based and fluro based techniques were compared with regards to accuracy and variability of component positioning. Currently we are able to use either a CT based system or a fluro based system to accurately insert the femoral component of the Oxford Unicompartmental Knee arthroplasty. Computer assist techniques are allowing us to perform minimally invasive arthroplasty procedures with great accuracy. Patients were all seen in a pre-admission clinic where pre-operative clinical survey data were collected. All patients had standing AP and lateral knee X-rays as well as skyline X-rays pre-operatively. Post-perative full length AP and lateral femur X-rays were completed in order to measure alignment of the femoral component. Patients are being followed post-operatively with SF-36, WOMAC, Knee Society Scores, and X-rays. Patients being operated on with the CT based system had pre operative CT scans. Intra-operatively a DRB was fixed to the patient’s femur and the chosen computer assisted technique was used to direct the rotation of the tibial cut as well as the alignment of the femoral cutting jig. To date we have completed seventeen computer assisted Oxford Unicompartmental Knee Arthroplasties. The average error in the AP plane using CT based system was 3.2 degrees and 2.1 degrees for the lateral plane. The average error in the AP plane using the fluro-based system was 2.2 degrees and 1.3 degree for the lateral plane. Funding: NSERC, IRIS, ORDCF

Ultra-high molecular weight polyethylene (UHMWPE) is a commonly used as bearing material in joint replacement devices. UHMWPE implants can be hard to see on a standard X-ray because UHMWPE does not readily attenuate X-rays. Radiopaque UHMWPE would enable direct imaging of the bearing both during and after surgery, providing in vivo assessment of bearing position, dislocation or fracture, and potentially a direct measure of wear. The X-ray attenuation of UHMWPE was increased by diffusing an FDA approved contrast agent (Lipiodol) into UHMWPE parts (Zaribaf et al, 2018). The aim of this study was to evaluate the optimal level of radiopacity for a UHMWPE bearing.

Samples of un-irradiated medical grade UHMWPE (GUR 1050) were machined into 4mm standard medium Oxford Unicompartmental bearings. Samples were immersed in Lipiodol Ultra Fluid (Guerbert, France) at elevated temperatures (85 °C, 95 °C and 105 °C) for 24 h to achieve three different levels of radiopacity.

A phantom set-up was used for X-ray imaging; the phantom contained two perspex rods to represent bone, with the metallic tibial tray and polyethylene bearing fixed to the end of one rod and the metallic femoral component fixed to the other rod. Radiographs of the samples were taken (n=5) with the components positioned in full extension. To ensure consistency, the images of all the samples were taken simultaneously alongside an untreated part.

The results of our ongoing study demonstrate that the radiopacity of UHMWPE can be enhanced using Lipiodol and the parts are visible in a clinical radiographs. The identification of the optimal treatment from a clinical perspective is ongoing; we are currently running a survey with clinicians to find the consensus on the optimal radiopacity taking into account the metallic components and alignment. Future work will involve a RSA study to assess the feasibility of measuring wear directly from the bearing.

Introduction

The number of medial unicompartmental knee replacements (UKR) performed for arthritis has increased and as such, revisions to total knee replacement (TKR) is increasing. Previous studies have investigated survivorship of UKR to TKR revision and functional outcomes compared to TKR to TKR revision, but have failed to detail the surgical considerations involved in these revisions. Our objectives are to investigate the detailed surgical considerations involved in UKR to TKR revisions.

Laboratory experiments and computational models were used to predict bone-implant micromotion and bone strains induced by the cemented and cementless Biomet Oxford medial Unicompartmental Knee Replacement (UKR) tibial implants.

Unicompartmental knee replacements (unis) offer an early option for the treatment of osteoarthritis. However there is no standard method for measuring the wear of unis in the laboratory. Most knee simulators are designed for TKA, for which there is an ISO standard. This study is about a wear method for unis, applied to a novel unicompartmental knee replacement (design by PSW). It has a metal-backed UHMWPE femoral component to articulate against a monoblock metallic tibial component. The advantage is reduced resection of strong bone from the proximal tibia for more durable fixation. The femoral component resurfaces the distal end of the femur to a flexion arc of only 42°, the area of cartilage loss in early OA (Fig. 1). We compared this novel bearing couple to the same design but with the usual arrangement of femoral metal and tibial plastic. Our hypothesis was that the wear of the reversed materials would be comparable to conventional and within the range of TKR bearings.

The test was conducted on a 4-station Instron-Stanmore force-controlled knee simulator. Both specimen groups (n=4 each) were highly crosslinked UHWMPE stabilized with vitamin E. On each of the four stations, one uni system was mounted on the medial side and one on the lateral, as if a standard TKR was being tested. The ISO-14243-1 walking cycle force-control waveforms were applied for 5 million cycles (Mc) at 1Hz, but with the maximum flexion during the swing phase (usually 58°) curtailed to 35° to maintain the contact within the arc of the femoral component. In-vivo this implant would be inlaid into the distal medial femoral condyle and the articulating surface immediately transitions into native cartilage. In our test set-up there was no secondary surface as such. The reduced flexion occurred during the swing phase where compressive load was low and the effect on the wear would be negligible. Wear was measured gravimetrically at many intervals and corrected by the weight gain of extra two active soak controls per group.

After 5 Mc, the average rates of gravimetric weight loss from the UHMWPE femoral and tibial bearings were 4.73±0.266 mg/Mc and 3.07±0.388 mg/Mc, respectively (statistically significantly different, p=0.0007) (Fig. 2). No significant difference was found in wear between medial and lateral placement for specimens of the same type, although the medial side generally wore more. Although the plastic femorals of the reverse design wore more than the plastic tibials, the wear was still low at <5 mg/Mc. The range for typical TKRs using ultra-high molecular weight polyethylene, tested under the same conditions in our laboratory has been 2.85–24.1 mg/Mc.

In summary, we adapted the ISO standard TKA wear test for the evaluation of unis, and in this case, a uni with reversed materials. Based on the wear results, this type of ‘early intervention’ design could therefore be a viable option, offering simplicity with less modular parts as well as load sharing with the native articular cartilage.

Introduction

The revision rate of unicompartmental knee replacement (UKR) in national joint registries is much higher than that of total knee replacements and that of UKR in cohort studies from multiple high-volume centres. The reasons for this are unclear but may be due to incorrect patient selection, inadequate surgical technique, and inappropriate indications for revision. Meniscal bearing UKR has well defined evidence based indications based on preoperative radiographs, the surgical technique can be assessed from post-operative radiographs and the reason for revision from pre-revision radiographs. However, for an accurate assessment aligned radiographs are required. The aim of the study was to determine why the revision rate of UKR in registries is so high by undertaking a radiographic review of revised UKR identified by the United Kingdom's (UK) National Joint Registry (NJR).

The purpose of this paper is to emphasize lateral compartment arthroplasty as efficient method and to discuss postoperative problems by investigating mid-term results (over 5 years)

Two hundred and twenty two consecutive unicompartmental knee arthroplasties (UKA) were performed in our hospital between August 1977 and December 1999. Thirty eight joints of 37 patients underwent lateral UKA. There were 8 male and 29 female patients with an average of 65.1 years old. The following prostheses were used: Marmor in 5 patients, Oxford in 1, PCA in 11, and Omnifit in 21. Out of them, 24 joints had follow up over 5 years after replacement. Three patients died of unrelated illness and 3 were lost to follow up. The remaining 18 joints could be followed from 60 to 189 months (average, 99 months). The knee score of the Hospital for Special Surgery (the HSS knee score) was used for clinical evaluation.

Clinical results by the HSS knee score showed that 2 joints were judged as “good”, 13 as “fair”, and 3 as “poor” before surgery. After surgery 13 joints were improved to “excellent”, 3 to “good”, and 2 to “fair”. The two joints with fair results had revision surgery. In all, 16 joints had satisfactory results. With radiological evaluation, the average preoperative alignment on standing was 14.9o of valgus angulation and corrected to 6.9 o of valgus. Although radiolucent lines under the tibial component were not found, a radiolucent line was observed and extended to loosening in one femoral component. This patient was successfully revised to another type of UKA 3 years after the index surgery. In the medial compartment, we found slight deterioration of osteoarthritic change in 5 joints. Of these, one patient, needed medial UKA.

Lateral UKA is a reliable and successful option compared with other procedures in a patient with low level of physical activity, because the long-term results tend to last and loosening of the tibial components has not been observed as commonly as in medial UKA. However, deterioration of the medial compartment may occur. The alignment after surgery must be in slight valgus for a satisfactory long-term result.

Introduction

Unicompartmental knee arthroplasty (UKA) has seen renewed interest in recent years and is a viable option for patients with limited degenerative disease of the knee as an alternative to total knee arthroplasty. However, the minimally invasive UKA procedure is challenging, and accurate component alignment is vital to long-term survival. Robotic-assisted UKA allows for greater accuracy of component placement and dynamic intraoperative ligament balancing which may improve clinical patient outcomes. The purpose of this study was to analyse the clinical outcomes in a large, consecutive cohort of patients that underwent robotic-assisted UKA at a single institution with a minimum follow-up of 2 years. The study hypothesis was that robotic-assisted UKA improves patient outcomes by decreasing the rate of revision in comparison to conventional UKA.

The Oxford mobile bearing knee prosthesis (Zimmer Biomet Inc, Warsaw, Ind) is considered a good treatment option for isolated medial compartment knee arthrosis. From February 2001 until August 2016, 1719 primary Oxford medial unicompartmental knee replacement procedures were completed at our center by a group of seven surgeons. We undertook this study to examine the long-term survivorship of the Oxford unicompartmental knee replacement looking at survivorship and reasons for failure.

A retrospective consecutive case series review was completed, and all revisions and re-operations were identified. Conversion to total knee replacement (TKA) was considered a failure. Kaplan-Meier survival analysis was used to calculate the 15-year survivorship of the group overall. We specifically looked at age, gender, BMI and surgeon caseload in addition to the reasons for failure. A statistical analysis was performed and differences in survivorship were compared for the variables listed. A logistic cox regression was performed to explore predictors of revision.

Overall 15-year survivorship was 89.9%. Female survivorship of 88.1% was statistically worse than the male group at 91.8% (p=0.018). Younger patients (75yrs of age (p= 0.036). There was a large range in surgical case load by individual surgeons (range 17–570 knees). There were no statistically significant differences in age, BMI, or gender when comparing the individual surgeon groups. There was a large range in 15-year survivorship between individual surgeons (range 78.3% – 95%). Overall the most common reason for revision was due to wear of the unreplaced portion of the knee (lateral and/or patella-femoral joint) followed by aseptic loosening, polyethylene dislocation, infection or persistent pain.

The 15-year survivorship results of the Oxford medial unicompartmental knee replacement at our center compares favourably to other published series and large registry data series. We found a reduction in survivorship in female patients and younger patients (< 5 5yrs). There were also significant differences in survivorship based on the individual surgeon. A more selective patient approach yielded the best long-term survivorship and equivalent to that of total knee replacement. We therefore suggest using a more selective approach when choosing patients for a medial unicompartmental knee replacement with the Oxford mobile bearing prosthesis in order to enhance long-term survivorship.

Although good long term results for fixed bearing uni-compartmental knee replacements (UKRs) have been reported mobile bearings predominate in some parts of the world. Three prospective studies have been undertaken comparing the short and medium term outcomes of fixed and mobile UKRs.

A 5 year comparative cohort study of 47 Oxford mobile bearing and 57 St Georg Sled fixed bearing UKRs.

In all groups the preoperative sex mix, average age and knee scores were extremely similar.

All patients were assessed both pre and postoperatively by a research nurse and radiographs were taken; the results were entered on the Bristol Knee database.

Results:

Multiple problems were encountered, perhaps because of the introduction of MIS, but at 5 years 11 Oxford and 4 Sleds had failed. The major problem with the mobile bearing implant was instability though tibial fractures were also seen. Both groups had three cases of arthritic progression and loose cement was seen twice in the fixed bearing group.

– Amongst the remaining patients the median scores for the Sled were better. Bristol Knee Score (Max 100) 95:90; Oxford (Max 48) 39:37; and reduced WOMAC (Best score 12) 18:24.

Conclusion. Although theoretically mobile bearings will give greater longevity there is frequently a short term price to pay particularly when the procedures are performed by inexperienced surgeons and trainees. Since the wear properties of polyethylene have improved in recent years these studies suggest that a fixed bearing option can be used more safely in many patients.

Unicompartmental knee replacements have been performed since the 1970’s. Controversy still exists as to the indications and contra-indications for these procedures, and there is still no clarity as to whether the patient should have a high tibial osteotomy, a unicompartmental knee replacement, or a total knee replacement. It has been suggested that unicompartmental knee replacements are preferable to high tibial osteotomies, as conversion to a total knee replacement is easier following a unicompartmental replacement.

Ten patients with unicompartmental knee replacements presented to the author requiring revision. All were revised to total knee replacements. In four a primary knee replacement could be performed, but the remaining six required a revision prosthesis on the tibial side, using stems and wedges. No revision prostheses were required on the femoral side.

Revision of a unicompartmental total knee replacement is technically easier than the revision of a total knee replacement. Revision of a high tibial osteotomy to a total knee replacement can be difficult, particularly if a poorly performed HTO had been done, with residual significant ligament imbalance.

The author feels that any type of revision surgery can be difficult. The author concludes that there is still no clarity as to whether one should do a unicompartmental knee replacement or a high tibial osteotomy, and that currently it is still the Surgeon’s choice as to which procedure he is going to perform.

Purpose of the study: The aim of this study was to report outcome after more than six years follow-up of a series of 122 unicompartmental prostheses.

Material and methods: Cemented HLS® unicompart-mental surface-coated prostheses were implanted in 111 patients (122 knees, 88% medial and 12% lateral) between January 1995 and November 1997 by the same surgeon. These knees presented unicompartmen-tal osteoarthritis (91%) or unicondylar necrosis (9%). An independent senior surgeon reviewed 94 prostheses. Seventeen patients died and ten institutionalized patients free of complaints about their knee could not be fully assessed. Only three patients (2.7%) were lost to follow-up). Clinical data were assessed with the IKS criteria. A complete radiological work-up was available to compare preoperative images with the last follow-up results. Mean follow-up was 88 months (range 72–108 months).

Results: After the implantation, 96% of patients were satisfied or very satisfied; 84% had no or little pain. Mean flexion was 133° (range 90–150°). The mean knee score at follow-up was 86/100 (40/100 preoperatively), mean function score 77/100 (61/100 preoperatively). Mean residual misalignment was 6° (7° varus for medial prostheses and 4° valgus for lateral prostheses). Tibial or femoral lucent lines were observed for 22% of the prostheses but with no change and no clinical expression. There was one case of tibial polyethylene wear (1mm). There were eight failures (all before 24 months) with revision with a total knee arthroplasty (two infections, one overlarge component, two tibial loosenings, 3 unexplained pain). The Kaplan-Meier survival at maximum follow-up of 108 months was 93.67%.

Discussion: The prostheses implanted in this series were correct indications according to the preceding symposiums. We analyzed the clinical and radiological outcome (overall axial correction, tibial and femoral correction), failures, and reasons for incomplete results.

Conclusion: Outcome at more than six years in this series of resurfaced knee prostheses with a polyethylene plateau was good, supporting the correct choice of implant and technique. These results also enabled validation of the principle that unicompartmental arthroplasty is a valid alternative for the treatment of unicompartmental osteoarthritis of the knee joint. Analysis of the failures and the incomplete results discloses interesting avenues for optimizing the surgical technique and improving future clinical and radiological results.

Study of failed Oxford medial unicompartmental knee replacements at the Royal Cornwall Hospital.

Joint load correlates with knee OA incidence, symptoms, radiographic, morphologic and biological changes. Available load modifying therapies are clinically effective but have drawbacks. The KineSpringTM (Moximed Inc), an investigational device, is designed to reduce compartment loads while avoiding the limitations of current treatments. We compare load reductions of braces, HTO and KineSpringTM.

Literature review and experimental data provide compartment load changes for clinically effective knee braces and HTO. Simulated gait testing was completed on four cadaver knees with early-stage OA. Gait was simulated using a cadaver-based kinematic test system that applies motion and loading patterns dynamically to cadaver specimens. Medial and lateral compartment femoro-tibial pressures were measured throughout testing using thin film dynamic pressure sensors (Tekscan, Inc.) placed inframeniscally. Three conditions were tested: no treatment, applied valgus moments to simulate a valgus moment brace, and implanted KineSpring.

Sufficient clinical data exists to support the development of new and novel load modifying therapies for knee OA. Joint load reductions provided by HTO and valgus moment braces provide insight into clinically effective load reduction ranges. Opening wedge HTOs of 5° and 10° are reported to reduce average medial compartment load by 55 N (12 lbs) and 286 N (64 lbs), respectively1. Valgus braces were reported to reduce medial compartment loads an average of 97-280 N (22-63 lbs). From this data we propose a clinically effective load reduction range of 55 to 286N is a valid indicator of the likely clinical success for medial knee load reduction treatments.

Gait simulation was successfully completed in all specimens in all test configurations. The valgus moment brace reduced medial compartment load by 58 ±20 N but did not reach statistical significance. The Kinespring reduced medial compartment load by 129±64 N in comparison to the untreated case, a statistically significant reduction. Neither the KineSpring nor the valgus moment brace caused significant changes in the lateral compartment during stance.

All treatments reduced medial compartment loads. KineSpringTM reduces loads in what we determined to be the clinically effective range. Additional studies and clinical investigations are warranted to determine the ultimate effectiveness of this implant system.