The shape of the distal femur: a geometrical study using MRI.

We scanned 25 left knees in healthy human subjects using MRI. Multiplanar reconstruction software was used to take measurements of the inferior and posterior facets of the femoral condyles and the trochlea. A 'basic circle' can be defined which, in the sagittal plane, fits the posterior and inferior facets of the lateral condyle, the posterior facet of the medial condyle and the floor of the groove of the trochlea. It also approximately fits both condyles in the coronal plane (inferior facets) and the axial plane (posterior facets). The circle fitting the inferior facet of the medial condyle in the sagittal plane was consistently 35% larger than the other circles and was termed the 'medial inferior circle'. There were strong correlations between the radii of the circles, the relative positions of the centres of the condyles, the width of the condyles, the total knee width and skeletal measurements including height. There was poor correlation between the radii of the circles and the position of the trochlea relative to the condyles. In summary, the condyles are approximately spherical except for the inferior facet medially, which has a larger radius in the sagittal plane. The size and position of the condyles are consistent and change with the size of the person. However, the position of the trochlea is variable even though its radius is similar to that of the condyles. This information has implications for understanding anterior knee pain and for the design of knee replacements.

A critique of revision rate as an outcome measure: re-interpretation of knee joint registry data.

National registers compare implants by their revision rates, but the validity of the method has never been assessed. The New Zealand Joint Registry publishes clinical outcomes (Oxford knee scores, OKS) alongside revision rates, allowing comparison of the two measurements. In the two types of knee replacement, unicompartmental (UKR) had a better knee score than total replacement (TKR), but the revision rate of the former was nearly three times higher than that of the latter. This was because the sensitivity of the revision rate to clinical failure was different for the two implants. For example, of knees with a very poor outcome (OKS < 20 points), only about 12% of TKRs were revised compared with about 63% of UKRs with similar scores. Revision therefore is not an objective measurement and should not be used to compare these two types of implant. Furthermore, revision is much less sensitive than the OKS to clinical failure in both types and therefore exaggerates the success of knee replacements, particularly of TKR.

Cementless Oxford unicompartmental knee replacement shows reduced radiolucency at one year.

We randomised 62 knees to receive either cemented or cementless versions of the Oxford unicompartmental knee replacement. The implants used in both arms of the study were similar, except that the cementless components were coated with porous titanium and hydroxyapatite. The tibial interfaces were studied with fluoroscopically-aligned radiographs. At one year there was no difference in clinical outcome between the two groups. Narrow radiolucent lines were seen at the bone-implant interfaces in 75% of cemented tibial components. These were partial in 43%, and complete in 32%. In the cementless implants, partial radiolucencies were seen in 7% and complete radiolucencies in none. These differences are statistically significant (p < 0.0001) and imply satisfactory bone ingrowth into the cementless implants.

Introduction of a new mobile-bearing total knee prosthesis: minimum three year follow-up of an RCT comparing it with a fixed-bearing device.

As part of the step-wise validation of a new prosthesis (TMK), we previously published the 1 year results of a randomised controlled trial in patients undergoing bilateral knee replacement [Price A., Rees J., Beard D., Juszczak E. et al. A mobile-bearing total knee prosthesis compared with a fixed-bearing prosthesis. JBJS B 2003;85-B-1:62-7.]. Forty patients had the new mobile-bearing prosthesis implanted in one knee and an established fixed-bearing device in the other (AGC). We now report the 3 year status of these patients and, in addition, review a separate multi-centre cohort of 172 patients who had undergone unilateral arthroplasty with the TMK. No significant differences were found in outcome (American Knee Society Score and Oxford Knee Score) between the two prostheses. The greater incidence of "clicking" in the mobile-bearing knee, reported in the previous review, persisted (TMK=48%, AGC=30%). The presence of this mechanical noise was found to have no relationship with outcome in either of the prostheses. The unilateral cohort study showed an acceptable complication rate for the new prosthesis, although some patients reported subjective instability. The method of controlled introduction of the TMK, of which this constitutes a further step, has allowed us to assess the significance of a reported problem (clicking) and to provide scientific data from which other surgeons can decide about use of the implant.

Development and clinical application of meniscal unicompartmental arthroplasty.

About one-third of osteoarthritic patients requiring knee replacement have focal lesions limited mainly to the medial compartment and can achieve excellent postoperative function after medial unicompartmental replacement. However, late failures of many unicompartmental prostheses require revision at a rate about twice that of total knee replacement. The use of a fully conforming mobile-bearing meniscal unicompartmental prosthesis in the hands of experienced surgeons can reduce revision rates to levels equivalent to the best results achieved with total knee replacement. The paper argues the case for such a prosthesis and demonstrates that the usual modes of failure of unicompartmental arthroplasty, most of them biomechanical, can thereby be avoided.

[Is computer navigation a useful tool in unicompartmental knee arthroplasty? A pilot cadaver study]. / Ist die Navigation bei der Schlittenprothese sinnvoll? Eine Pilotstudie am Leichenknie.

We conducted this pilot cadaver study to investigate whether the use of a navigation system during minimally invasive unicompartmental knee arthroplasty leads to more consistent results than the conventional hand-guided technique. We describe the accuracy of implant positioning in using standard instrumentation and computer navigation. Radiographic assessment showed that accurate component placement was achieved using both methods. These results were not statistically significant. The computer navigated femoral component placement without intramedullary (IM) rod was as accurate as the conventional method with IM rod. The study showed that computer navigation can produce accurate results even without an intramedullary rod. Image guidance can maintain the accuracy of the standard instrumentation and enhance 3D vision and the intraoperative orientation of the surgeon.

[Medial unicompartmental knee replacement using the "Oxford Uni" meniscal bearing knee]. / Die mediale Schlittenprothese mit mobilem Polyethylenmeniskus "Oxford Uni".

Medial unicompartmental knee replacement (UKR) has many advantages over total replacement (TKR) including better function and reduced morbidity. However, the long-term failure rates of fixed-bearing UKR are high, especially because of polyethylene wear. The fully congruent mobile bearing of the Oxford UKR exhibits minimal polyethylene wear, failure from this cause does not seem to occur before 10 years. The instrumentation allows precise implantation to restore isometric function of the ligaments. During its 20 years development, the limits of usefulness of the implant have been established and found to include about one in four knees requiring replacement for osteoarthritis. In an independent series, using these criteria, the 15 year survival was 94%. Since 1998, the phase 3 implant has been used with modified instruments through a small incision, avoiding damage to the extensor mechanism. Patients now recover about three times faster than after TKR, and regain much better flexion (mean 135 degrees ). The current evidence supports that the minimally invasive Oxford UKR should be seriously considered as primary treatment for anteromedial compartment osteoarthritis-provided the appropriate surgical expertise is available.

A mobile-bearing total knee prosthesis compared with a fixed-bearing prosthesis. A multicentre single-blind randomised controlled trial.

Before proceeding to longer-term studies, we have studied the early clinical results of a new mobile-bearing total knee prosthesis in comparison with an established fixed-bearing device. Patients requiring bilateral knee replacement consented to have their operations under one anaesthetic using one of each prosthesis. They also agreed to accept the random choice of knee (right or left) and to remain ignorant as to which side had which implant. Outcomes were measured using the American Knee Society Score (AKSS), the Oxford Knee Score (OKS), and determination of the range of movement and pain scores before and at one year after operation. Preoperatively, there was no systematic difference between the right and left knees. One patient died in the perioperative period and one mobile-bearing prosthesis required early revision for dislocation of the meniscal component. At one year the mean AKSS, OKS and pain scores for the new device were slightly better (p < 0.025) than those for the fixed-bearing device. There was no difference in the range of movement. We believe that this is the first controlled, blinded trial to compare early function of a new knee prosthesis with that of a standard implant. It demonstrates a small but significant clinical advantage for the mobile-bearing design.

Rapid recovery after oxford unicompartmental arthroplasty through a short incision.

Forty Oxford medial unicompartmental arthroplasties (UCAs) were performed through a short incision medial to the patellar tendon, without dislocation of the patella, using updated instruments (Oxford Knee Phase III, Biomet Ltd., Bridgend, UK). The rate of recovery of these knees (measured by the time taken to achieve straight-leg raising, 70 degrees of flexion, and independent stair climbing) was compared with that of 20 Oxford UCAs performed through an open approach with dislocation of the patella. Both groups were compared with 40 AGC (Biomet) total knee arthroplasties performed for osteoarthritis during the same time period. The average rate of recovery after the short-incision UCA was twice as fast as after open UCA and 3 times as fast as after total knee arthroplasty. Accuracy of implantation, assessed from 11 variables seen on fluoroscopically centered postoperative radiographs, was the same after UCA by the short-incision approach as after the open approach, suggesting that the short-term advantage of increased speed of recovery was gained without affecting the long-term results. We now employ the short incision with the phase III instruments for all UCAs.

Does arthritis progress in the retained compartments after 'Oxford' medial unicompartmental arthroplasty? A clinical and radiological study with a minimum ten-year follow-up.

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.

The Oxford medial unicompartmental arthroplasty: a ten-year survival study.

Retrieval studies have shown that the use of fully congruent meniscal bearings reduces wear in knee replacements. We report the outcome of 143 knees with anteromedial osteoarthritis and normal anterior cruciate ligaments treated by unicompartmental arthroplasty using fully congruous mobile polyethylene bearings. At review, 34 knees were in patients who had died and 109 were in those who were still living. The mean elapsed time since operation was 7.6 years (maximum 13.8). We established the status of all but one knee. There had been five revision operations giving a cumulative prosthetic survival rate at ten years (33 knees at risk) of 98% (95% CI 93% to 100%). Considering the knee lost to follow-up as a failure, the 'worst-case' survival rate was 97%. No failures were due to polyethylene wear or aseptic loosening of the tibial component. One bearing which dislocated at four years was reduced by closed manipulation. The ten-year survival rate is the best of those reported for unicompartmental arthroplasty and not significantly different from the best rates for total knee replacement.

In vitro measurement of patellofemoral force after three types of knee replacement.

Using a new, non-invasive method, we measured the patellofemoral force (PFF) in cadaver knees mounted in a rig to simulate weight-bearing. The PFF was measured from 20 degrees to 120 degrees of flexion before and after implanting three designs of knee prosthesis. Medial unicompartmental arthroplasty with a meniscal-bearing prosthesis and with retention of both cruciate ligaments caused no significant change in the PFF. After arthroplasty with a posterior-cruciate-retaining prosthesis and division of the anterior cruciate ligament, the PFF decreased in extension and increased by 20% in flexion. Implantation of a posterior stabilised prosthesis and division of both cruciate ligaments produced a decrease in the PFF in extension but maintained normal load in flexion. There was a direct relationship between the PFF and the angle made with the patellar tendon and the long axis of the tibia. The abnormalities of the patellar tendon angle which resulted from implantation of the two total prostheses explain the observed changes in the PFF and show how the mechanics of the patellofemoral joint depend upon the kinematics of the tibiofemoral articulation.

In vitro patellofemoral joint force determined by a non-invasive technique.

OBJECTIVE: To develop a method of measuring the magnitude, direction and point of application of the patellofemoral force (PFF), directly and non-invasively in three dimensions. DESIGN AND METHODS: The compressive PFF is replaced exactly with a tensile force applied to the front of the patella. The magnitude, direction and point of application of the tensile force are then measured. The technique was applied to six normal knees mounted in a 6 degree of freedom rig with quadriceps tendon tension force (QTF) applied to balance a flexing load and to simulate weight bearing. RESULTS: The PFF was greater than in previous more invasive in vitro studies but the results correlated well with recent theoretical analyses. At 20 degrees knee flexion the force was 75% of QTF. It increased to 100% of QTF at 60 degrees knee flexion and remained at this level at higher angles of flexion. The lateral vector of the PFF was small compared to the sagittal plane vector and became negligible beyond 60 degrees of knee flexion. The point of application of the PFF to the patella moved proximally and medially with knee flexion. CONCLUSIONS: A new and reliable method of measuring PFF non-invasively and in three dimensions has been developed. RELEVANCE: A new technique is described for measuring the PFF in vitro. The non-invasive nature of the technique makes it useful for studying the effect on the PFF of simulated pathological conditions, surgical procedures and different types of knee replacement.

Theory and practice of meniscal knee replacement: designing against wear.

This paper attributes the high rate of high-density polyethylene (HDP) wear in many knee prostheses to incongruity of their articular surfaces. The authors discuss the anatomical and kinematic features of the knee which have led most designers to choose incongruous surfaces and review the reasons for employing free bearings, analogues of the natural menisci, to provide congruity without restriction of movement. There are theoretical reasons against employing freely mobile bearing in the absence of a functioning anterior cruciate ligament (ACL) and the authors' experience has confirmed this limitation in practice. It is concluded that, in bicompartmental replacement, the movements of the meniscal bearing should be limited in the anteroposterior direction. For unicompartmental arthritis, an unconstrained bearing can be employed if, as is usually the case, all ligaments are intact.

Bilateral knee replacements: simultaneous or staged?

Approximately one-third of primary knee replacements are performed on patients with bilateral symptomatic disease. This retrospective study compared the complication rate, functional outcome and cost implications of performing bilateral total knee replacements under a single anaesthetic or as staged procedures with a control group of unilateral cases. There was no significant difference in the complications or outcome. There was a significant reduction in the mean hospital admission time of 7 days per knee replacement (P < 0.001, Student's t test) by performing bilateral knee replacements for osteoarthritis under the same anaesthetic. However, a survey of practising knee surgeons in three health regions in the United Kingdom shows that only 24% would regularly perform bilateral knee replacements under one anaesthetic.

The radiographic classification of medial gonarthrosis. Correlation with operation methods in 200 knees.

Ahlbäck's classification of gonarthrosis can be applied with improved precision by careful interpretation of anteroposterior varus stress and lateral radiographs of the knee. The tibial lesion in early gonarthrosis is located in the anterior and middle part of the medial plateau. In more advanced disease, when the anterior cruciate ligament is invariably damaged, the lesion extends to the posterior margin of the medial tibial plateau. We studied the preoperative radiographs of 200 knees with arthrosis. We were able to predict the integrity of the anterior cruciate ligament (and the use of unicompartmental arthroplasty) with 95 percent accuracy and rupture or damage to the anterior cruciate ligament (and the use of total condylar arthroplasty) with 100 percent accuracy. We believe that the Ahlbäck classification reflects the anatomic and pathologic progression of medial compartment gonarthrosis, and is of value in allowing more accurate comparisons to be made of different methods of treatment.

Anteromedial osteoarthritis of the knee.

Medial tibial plateaux excised during 46 unicompartmental arthroplasties for osteoarthritis were collected and photographed. The anterior cruciate ligament was intact in all joints. In every case the cartilage and bone erosion was centred anteriorly on the plateau and the posterior cartilage was intact. The site of the lesion and the intact state of the cruciate ligaments taken together explain why varus deformity was observed only in the extended knee, and why the deformity was correctable and had not become fixed. Failure of the anterior cruciate ligament may allow the erosion to extend posteriorly, producing fixed varus deformity and leading to degeneration of the lateral compartment. Anteromedial osteoarthritis is a distinct clinicopathological entity; its radiographic features enable it to be diagnosed from lateral radiographs; its anatomical features render it suitable for treatment by unicompartmental arthroplasty.

Sagittal plane laxity following knee arthroplasty.

We measured the sagittal laxity in 70 knee replacements at least six months after surgery, using a KT 1000 arthrometer. With an unconstrained prosthesis (the Oxford meniscal knee) anteroposterior stability was normal in joints known to have intact cruciate ligaments. There was increased laxity in those which lacked an anterior cruciate ligament. In knees with an intact anterior cruciate ligament, sagittal laxity did not increase with time.