Bioactive Bone Cement: The Solution for Osteolysis and Late Implant Loosening

Introduction: High failure rates have been associated with spinal instrumentation in osteoporotic vertebrae. Polymethylmethacrylate (PMMA) augmentation of thoracic pedicle screws doubles the initial pull out strength, however, the microscopic layer of fibrous tissue which develops at the bone/cement interface with standard PMMA may lead to late loosening and construct failure. Inflammatory mediators have been linked to this process. Barium , which is added to standard PMMA for radiopacity, potentates these effects and is itself osteolytic. It comprises 10% by weight of standard PMMA. Bioactive cements and additives such as hydroxyapatite pastes/coatings, carbonated apatite cement, and glass-ceramic powders are currently being investigated to address the issues of inflammation and loosening.

Purpose: To develop a bioactive bone cement which promotes osseous ingrowth/apposition that can be substituted for standard PMMA to augment spinal istrumentation, perform kyphoplasty, and assist in reconstruction of anterior column defects created during tumor surgery.

Materials and Methods: Pure PMMA (without barium) was mixed with increasing concentrations of hydroxyapatite (HA) [10%, 15%, 20%, 25%, 30%]. Each group of specimens were mechanically tested in accordance with the ASTM specifications using tensile bars to determine the maximum percent composition by weight of HA that could be added to pure PMMA without compromising its strength as compared to standard PMMA. Each group was evaluated with X-rays and compared to standard PMMA for degree of opacification. Dowels of either pure PMMA mixed with 20% HA or standard PMMA were surgically implanted into the proximal tibia and distal femur of each hindquarter in six sheep. The sheep were harvested at three and six months. Histologic examination was performed on the explanted specimens. The bone/cement interfaces were evaluated to determine the density of bone within a 2mm zone around the implants and the percentage of fibrous tissue apposition around the circumference of the dowels.

Results: The mechanical strength of the HA/PMMA cement compared to the standard PMMA was not statistically different at HA concentrations < 20% (p = 0.51). Equivalent radiopacity was achieved by exchanging the barium for hydroxyapatite. The density of the cancellous bone surrounding the HA/PMMA dowels was significantly greater than the density of bone surrounding the standard PMMA dowels (p = 0.0018) at both three and six month. Additionally, there was significantly more fibrous tissue surrounding the standard PMMA dowels than the HA/PMMA dowels (p = 0.022).

Discussion: Pure polymethylmethacrylate without barium can be mixed with hydroxyapatite to form a “bioactive” cement that is mechanically and radiographically equivalent to standard PMMA. This cement results in a more exuberant osseous response surrounding the dowels (p=0.0018) with less fibrous tissue apposition (p=0.022) than standard PMMA. Because the HA/PMMA combination promotes the development of dense bone surrounding the cement and lacks barium, which is osteolytic, there may be a decreased risk of osteolysis and subsequent loosening with the use of HA/PMMA as opposed to standard PMMA. This data shows promise in developing a simple yet effective “bioactive” replacement for standard PMMA.