M. A. R. Freeman

Tibiofemoral movement 2: the loaded and unloaded living knee studied by MRI

In 13 unloaded living knees we confirmed the findings previously obtained in the unloaded cadaver knee during flexion and external rotation/internal rotation using MRI. In seven loaded living knees with the subjects squatting, the relative tibiofemoral movements were similar to those in the unloaded knee except that the medial femoral condyle tended to move about 4 mm forwards with flexion. Four of the seven loaded knees were studied during flexion in external and internal rotation. As predicted, flexion (squatting) with the tibia in external rotation suppressed the internal rotation of the tibia which had been observed during unloaded flexion.

Observations upon the interface between bone and polymethylmethacrylate cement

The occurrence of a radiolucent line at the interface of bone and cement in total joint prostheses is a frequently observed, although little understood, phenomenon. Because of an operative technique utilised in instances of bone loss, we have, within a single implant mass used in each of a series of 18 total knee replacements, been able to observe two separate interfaces, one between bone and cement and the other between bone and cobalt chrome. The average period of observation was 32 months. All of the knees except one demonstrated a lucency at the bone-cement interface; only one of the knees had a similar lucency at the bone-CoCr interface. One of the knees was studied histologically. In the light of the universal observation of macrophages at bone-cement interfaces and the recent finding that osteoclasts are derived from macrophages, these observations are significant in relation to the aetiology of bone-cement lucencies.

One-stage reimplantation for infected total knee arthroplasty

One-stage reimplantation for the salvage of infected total knee arthroplasty in 18 patients was reviewed at an average follow-up of five years. There had been one recurrence and one new infection, both in rheumatoid patients with another focus of infection. In four other patients the clinical result was impaired by pain after walking (2) and limited flexion (2). Our results suggest that one-stage reimplantation is a reasonably reliable procedure for the management of a loose infected prosthesis.

EARLY RADIOLOGICAL OBSERVATIONS MAY PREDICT THE LONG-TERM SURVIVAL OF FEMORAL HIP PROSTHESES

We reviewed a consecutive series of 527 uninfected hip replacements in patients resident in the UK which had been implanted from 1981 to 1993. All had the same basic design of femoral prosthesis, but four fixation techniques had been used: two press-fit, one HA-coated and one cemented. Review and radiography were planned prospectively. For assessment the components were retrospectively placed into two groups: those which had failed from two years onwards by aseptic femoral loosening and those in which the femoral component had survived without revision or recommendation for revision. All available radiographs in both groups were measured to determine vertical migration and examined by two observers to agree the presence of radiolucent lines (RLLs), lytic lesions, resorption of the neck, proximal osteopenia and distal intramedullary and distal subperiosteal formation of new bone. We then related the presence or absence of these features and the rate of migration at two years to the outcome with regard to aseptic loosening and determined the predictive value of each of these variables. Migration of > or = 2 mm at two years, the presence of an RLL of 2 mm occupying one-third of any one zone, and subperiosteal formation of new bone at the tip of the stem were predictors of aseptic loosening after two years. There were too few lytic lesions to assess at two years, but at five years a lytic lesion > or = 2 mm also predicted failure. We discuss the use of these variables as predictors of femoral aseptic loosening for groups of hips and for individual hips. We conclude that if a group of about 50 total hip replacements, perhaps with a new design of femoral stem, were studied in this way at two years, a mean migration of < 0.4 mm and an incidence of < 10% of RLLs of 2 mm in any one zone would predict 95% survival at ten years. For an individual prosthesis, migration of < 2 mm and the absence of an RLL of < or = 2 mm at two years predict a 6% chance of revision over approximately ten years. If either 2 mm of migration or an RLL of 2 mm is present, the chances of revision rise to 27%, and if both radiological signs are present they are 50%. If at five years a lytic lesion has developed, whatever the situation at two years, there is approximately a 50% chance of failure in the following five years. Our findings suggest that replacements using a limited number of any new design of femoral prosthesis should be screened radiologically at two years before they are generally introduced. We also suggest that radiographs of individual patients at two years and perhaps at five years should be studied to help to decide whether or not the patient should remain under close review or be discharged from specialist follow-up.

A correlative study of the geometry and anatomy of the distal femur.

Sixteen knees were examined roentgenographically in the lateral plane. Ten knees were examined from autopsy subjects. The distal articular femur may be represented by three circular surfaces: (1) the floor of the patellar groove (articulating with the patella from 10 degrees to 100 degrees), (2) the posterior femoral condyles (articulating with the tibia from 10 degrees to 150 degrees), and (3) the distal condyles (articulating with the tibia from 0 degrees to 10 degrees). The radii of these surfaces, their angular arcs, and the distances between their centers varied with the size of the femur but fell within a narrow range. The radii of the patellar groove and the posterior femoral condyles averaged 24 mm and 21 mm, and the average angle subtended by these arcs was 90 degrees and 140 degrees, respectively. The average distance between the centers of these two circles was 20 mm. The femoral attachment of the synovial and patellar retinacular reflections was found in the area of the center of the patellar groove circle. The femoral attachments of the medial collateral and posterior cruciate ligaments and of the lateral collateral and anterior cruciate ligaments were found to be in the area of the center of the circle of the medial and lateral posterior femoral condylar circles, respectively.

The size and shape of particulate polyethylene wear debris in total joint replacements.

Abstract Osteolysis induced by wear particles has been recognized as one of the major causes of long-term failure in total joint replacements. However, little is currently known about the exact nature of particles, as the particles are too small to be characterized by light microscopy. In this study, ultra-high molecular weight polyethylene (UHMWPE) particles retrieved from ten cases (six cemented and four uncemented) for Freeman type conforming tibiofemoral total knee replacements (TKRs), three Charnley total hip replacements (THRs) and five Imperial College/London Hospital double cup surface hip replacements for aseptic loosening were extracted using a high-performance method with ultracentrifugation and characterized by scanning electron microscopy. The equivalent circle diameter (ECD) of all 18 cases ranged from 0.40 to 1.15 μm (Mean ± SE = 0.70 ± 0.05 μm, median = 0.67 μm). The aspect ratio was 1.50 to 2.04 (Mean ± SE = 1.75 ± 0.04, median = 1.73), and roundness was 1.24 to 2.34 (Mean ± SE = 1.61 ± 0.07, median = 1.65). The numbers of particles were 5.2 × 108 to 9.17 × 1010/g tissue (Mean ± SE = 1.42 × 1010 ± 5.41 × 109/g tissue, median = 7.04 × 109). The number of polyethylene (PE) particles/g tissue in TKRs was significantly larger than that in THRs (1.04 × 1010/g tissue and 2.16 × 109/g tissue respectively, median. p = 0.03, Mann-Whitney U test). Unstable fixation of the tibial PE component might account for the accumulation of a large number of PE particles in the interface tissue.

Wear particulate species and bone loss in failed total joint arthroplasties

The aim of this study was to evaluate the relative contribution of polyethylene, metal, and polymethylmethacrylate (cement) particles to the overall bone loss in aseptic loosening. Twenty-four interface tissues with adjacent bone were obtained during 17 revision total joint arthroplasties (11 hips and six knees). Osteoclasts and macrophages were identified immunohistochemically on the bone surface. The length of the bone surface in contact with these cell types was measured and analyzed with reference to the particulate species present within the fibrous interface. The presence of abundant polyethylene particles significantly increased the proportion of the bone surface in contact with macrophages but did not have a significant influence on that of osteoclasts. Osteoclastic bone resorption was significantly more extensive in the presence of metal particles. In contrast, the presence of cement particles did not have a significant influence on macrophage or osteoclast coverage of the bone surface. These results highlight the significance of polyethylene particles in macrophage recruitment and subsequent osteolysis and suggest a different mechanism of bone loss related to metal, namely mediation through osteoclastic activities. The relative contribution of cement particles was negligible and needs reevaluation in light of evidence provided by others.

RADIOLOGICAL AND SURVIVAL COMPARISON OF FOUR METHODS OF FIXATION OF A PROXIMAL FEMORAL STEM

We compared the radiological appearances and survival of four methods of fixation of a femoral stem in 538 hips after follow-up for five or ten years. The fixation groups were: 1) press-fit shot-blasted smooth Ti-Al-V stem; 2) press-fit shot-blasted proximally ridged stem; 3) proximal hydroxyapatite (HA) coating; and 4) cementing. Survival analysis at five to ten years showed better results in the HA-coated (100% at five to six years) and cemented stems (100% at 5 to 6 years) than in the two press-fit groups. There was a higher mean rate of migration in the smooth and ridged Ti-Al-V shot-blasted press-fit groups (0.8 mm/year and 0.6 mm/year, respectively) when compared with the HA-coated and cemented prostheses (both 0.3 mm/year). More radiolucent lines and osteolytic lesions were seen in the press-fit groups than in either the HA-coated or cemented implants, with a trend for a lower incidence of both in the HA compared with the cemented group. Proximal osteopenia increased in the press-fit and cemented prostheses with time, but did not do so in the HA group. There was a higher incidence of resorption of the femoral neck with time in the cemented group than in the other three. We conclude that the HA and the cemented interfaces both provide secure fixation with a trend in favour of HA. The cemented prosthesis meets the suggested National Institutes of Health definition of ‘efficacious’ at ten years.

Tibiofemoral movement 4: changes of axial tibial rotation caused by forced rotation at the weight-bearing knee studied by RSA.

We studied the knees of 11 volunteers using RSA during a step-up exercise requiring extension while weight-bearing from 50 degrees to 0 degrees. The findings on weight-bearing flexion with and without external rotation of the tibia based on MRI were confirmed.

Early migration of prostheses related to long-term survivorship. Comparison of tibial components in knee replacement

We determined the cumulative survival rates, in arthroplasty of the knee, of three designs of tibial component, using a change of position on standard radiographs or revision for aseptic loosening as criteria of failure. The average migration of each of the three designs in the first postoperative year is known from roentgen stereophotogrammetric analysis reported by other authors. The ranking order of the components as judged by cumulative survival is the same as that determined by early migration. This finding supports the view that the measurement of early migration can predict late aseptic loosening and therefore that such measurements are clinically of value.