Thermal damage to bone related to the exothermic polymerisation of bone cement (PMMA) remains a concern. A series of studies were conducted to examine PMMA bone interface during cemented arthroplasty.

In vitro and in vivo temperature distributions were performed in the laboratory and human and animal surgery. In vivo (10 patients) measurements of cement temperature during cementing of BHR femoral prosthesis using thermocouples. Intra-operative measurement of cement temperature in BHR in the presence of femoral head cysts was examined in patients. The BHR femoral heads were sectioned to assess cement mantle as well as position of thermocouples. An additional study was performed in sheep with PMMA implanted into cancellous defects. Thermocouples were used to monitor temperature in the cement as well as adjacent bone. Histology and CT was used to assess any thermal damage.

The exothermic reaction of PMMA during polymerization does indeed result in an increase in temperature at the interface with bone. The in vivo study recorded a maximum temperature of 49.12C for approximately three minutes in the cancellous bone underneath the BHR prosthesis. This exposure is probably not sufficient to cause significant injury to the femoral head. The maximum temperature of the cement on the surface of the bone was 54.12C, whereas the maximum recorded in the cement in the mixing bowl was 110.2C.

In the presence of artificial cysts within the bone, however, temperatures generated within the larger cysts, and even at the bone-cement interface of these cysts, reached levels greater than those previously shown to be harmful to bone. This occurred in one case even in the 1 cc cyst.

Routine histology revealed a fibrous layer at the cement bone interface in the sheep study. Fluorescent microscopy demonstrated bone label uptake adjacent to the defect site. Histology did not reveal thermal necrosis in the defects in terms of bony necrosis. CT data was used to measure the amount of PMMA placed into each defect. This analysis revealed a range of volumes that did not seem to influence the histology.

The heat of cement polymerisation in resurfacing as performed in our study is not sufficient to cause necrosis. This may reflect the ability of the body to rapidly conduct heat away by acting as a heat sink. The temperature-conducting properties of the metal prosthesis are also likely to be important.

Introduction: Prevalence of femoral neck fracture in resurfacing hip arthroplasty continues to question if failure is technique-related or due to the inherent bone quality. This study aimed to correlate cement penetration profile during resurfacing hip with inherent bone density. The hypothesis is that osteoporotic bone is unable to support the prosthesis leading to fatigue failure.

Methods: Fifteen patients scheduled for total hip replacement (THR) were recruited to undergo resurfacing arthroplasty prior to THR. Each patient was implanted with a resurfacing femoral component (BHR, Smith & Nephew, Memphis, TN). Antibiotic simplex cement was inserted one minute after mixing at 18°C to fill 10% of the femoral component volume. The femoral head-implant section was removed and kept in buffered formalin. The patients then proceeded with standard THR. The femoral head-cement-prosthesis section was separated using electrical discharge (ED) machining technique and CT-scanned. The depth and volume of cement penetration were measured from the CT scans and correlated with femoral neck bone densities.

Results: Cement penetration was compared for three groups of bone density: normal, osteopenic, osteoporotic. Average cement thickness were found to be 0.36 ± 0.16mm (proximal), 0.28 ± 0.11 mm (centre) and 0.12 ± 0.05 mm. During hip resurfacing, cement is forced into the porous structure, e.g. the trabeculae and airspaces when the femoral component is fixed onto the head of the femur. In normal bone, the trabeculae is dense and air spaces occupy a small volume of the bone. Greater cement penetration was expected in osteopenic and osteoporotic bones. However, no significant difference was found between cement thickness and volume against inherent patient bone density (p> 0.05). High viscosity of the cement may have prevented more cement to penetrate the bone. While the exterior cortex of the femoral head is strengthened by a cement layer, the interior structure of the femoral neck is still susceptible to fracture at high loads. In addition, increased bone necrosis due to the exothermic reaction during cement fixation may predispose patients to fracture.

Discussion: Resurfacing hip replacement is a viable technique if the fracture risk can be reduced by gaining the best possible cement penetration. This would provide continuous cement stiffness with the bone.