Trauma

Basic Science

Fixation Augmentation Techniques
Antonio Moroni, MD
November, 2005

When managing fragility fractures, the diagnosis of osteoporosis is the first hurdle that the orthopaedic surgeon encounters. Among the several diagnostic tools available, dual energy xray absorptiometry (DXA) scans have been identified as the gold standard since 1994 by the World Health Organization. Regrettably, when treating elderly fracture patients, the use of DXA scans prior to surgery is not common practice. This imposes a limitation for the orthopaedic surgeon who is lacking crucial data concerning patient bone quality. We believe that knowing patient bone quality prior to surgery would be useful not only for the administration of the correct medical therapy which is effective for secondary fracture prevention but also for selecting the correct surgical treatment, including the use of fixation augmentation techniques. Fixation augmentation techniques can be defined as surgical procedures aimed at increasing implant stability. These techniques include a variety of biological and orthobiological materials such as bone grafts, polymethylmethacrylate (PMMA), calcium phosphate implants, calcium phosphate cements, calcium phosphate coatings and modified implants (1). Recently, innovative “pharmaceutical” augmentation concepts such as bisphosphonates have also been introduced.

Bone Grafts

Bone grafting plays a significant role when treating osteoporotic fractures. Both cancellous and corticocancellous bone grafts have been used to enhance fracture healing of osteoporotic fractures. Cancellous grafts, for example bone chips, yield little mechanical strength and are generally used to either fill a void or improve the fractured host bone after fixation is complete with the purpose of stimulating new bone formation periosteally. Corticocancellous grafts yield significant mechanical strength and can be used to either replace bone losses or to augment the mechanical stability of the fixation. The use of cancellous grafts therefore involves a biological concept while the use of corticocancellous grafts involves a biological as well as a mechanical concept.

Grafts can be autogenous or allografts. Autogenous bone marrow has been used clinically to augment the osteogenic response to implanted allografts. In osteoporotic fractures, autogenous cancellous bone grafts are generally derived from freshly harvested bone marrow from the iliac crest and combined with the other materials such as osteoconductive substrates. Rates of donor-site morbidity associated with this harvest can be as high as 25% (2). It is interesting to report that there is no evidence that osteoporotic bone graft is an inferior graft material to normal healthy bone. This confirms the normal healing potential of osteoporotic bone. With osteoporotic fracture care, the challenge is the fixation. Cancellous grafts consist of osteoinductive, osteoconductive and osteogenic cells (3). These features are conducive to bone growth in areas needing restoration and repair. Bone grafts contain hydroxyapatite and collagen which serve as an osteoconductive platform, osteogenic stromal cells and growth factors such as bone morphogenic protein (BMP) and transforming growth factor-beta, which contribute to bone regeneration and promote fracture healing.

Corticocancellous bone grafts can either be nonvascularized or vascularized. Studies have shown that vascularized bone grafts are superior to non-vascularized grafts particularly when the bridging is more than 12cm (4). Vascularized bone grafts have a greater ability to heal stress related fractures while providing structural integrity. The disadvantages of this approach, however, include longer operative times and donor-site morbidity. In the osteoporotic patient, corticocancellous allografts should also be considered because it has been shown that they facilitate bone healing. The major advantage of these allografts is the absence of donor-site morbidity. Corticocancellous allografts have the clear advantage of being histocompatible. Furthermore, the disease transmission incidence is negligible. In one well-documented instance of a donor with AIDS, grafts that were lyophilized and irradiated did not give rise to AIDS, while fresh frozen grafts did. There is therefore a suggestion that such processing of bone may destroy the AIDS virus. (4).

Allografts can be used in a variety of fracture types to elevate the joint. A typical example is tibial plateau fractures where corticocancellous grafts can be inserted under the depressed joint line, restoring the joint anatomy and improving fracture stability. Allografts can also be used in long-bone fractures, opposite to plate fixation to increase screw purchase. This has been done in several fracture types both in the upper and lower extremities (5).

Bone cements

There are two kinds of bone cements for orthopaedic procedures, conventional cement (PMMA) and bioasorbable composites. Cement augmentation is a very useful technique for fixation of implants such as screws, plates, or IMnails in osteoporotic bone. Generally, cement augmentation features increased strength of implant fixation, rapid restoration of patient mobility, and reduction of the complications of implant failures. The occurrence of severe skeletal loss requires the stability provided by polymethylmethacrylate (PMMA). In a biomechanical study which investigated the treatment of osteoporotic trochanteric patients, femurs which had the lowest bone density showed a significant improvement in the initial fixation stability and total displacement of the femoral head was reduced by 39% on average in the group with the modified sliding hip screw (SHS) and PMMA compared to the standard SHS group (6). PMMA is however a permanent implant which has its shortcomings and complications such as excessive heat generation leading to soft tissue and bone damage. Because of this however, its routine use with sliding-hip-screw fixation for nonpathological fractures is currently not recommended.

Besides PMMA, bioabsorbable cements such as Norian SRS cement and Bone Source (How medica, Leibinger, Dallas, TX) have also been used for augmenting implants. Alternatively, cements which are made from calcium phosphate have a better purchase to bone and supersede PMMA as void fillers. Cements that are resorbable can be tailored to provide sufficient rigidity to allow bone healing and retain mechanical strength for a period of time. Subsequently the cement undergoes degradation and is eventually replaced by the host bone. Another advantage of using cements is the possibility of early weight bearing. The advantage of cementing screws is the intimate screw-cement interface, which increases the holding power. An innovative screw design which facilitates PMMA augmentation is the ported screw. Bone cement can be injected into the screw shaft and then flows via side holes into the surrounding bone. The holding power was 278% greater than the holding power of a standard screw (7).

Ceramic blocks

Ceramics are available in blocks which alone confer little bending-strength, shear or tension until well incorporated into the bone. The blocks must be used in conjunction with an autograft or have access to rich bone marrow because they are exclusively osteoconductive. These are effective graft fillers or expanders and are an excellent choice in compression and for the provision of structural support.

Coated screws

Pin fixation problems are particularly severe in osteoporotic bone and the achievement of superior screw fixation is critical for patients with osteoporosis. Coatings with hydroxyapatite have been developed to improve osteointegration and consequently the mechanical stability of the bone-pin interface. In a previous study, twenty female osteoporotic wrist fracture patients were selected (8). Patients were divided into two paired groups and randomized to receive either standard (uncoated) tapered pins (Group A) orhydroxyapatite-coated tapered pins of the same diameter (Group B). A Pennig II wrist fixator was used in all the patients (Orthofix srl, Bussolengo, Italy). Two pins were implanted in the distal radius and two in the second metacarpal. Pin insertion torque was measured during implantation. All frames were removed 6 weeks after surgery. Pin extraction torque was measured at the end of treatment. Average patient age was 75 ± 7 years in Group B. Mean final pin insertion torque was 461 ± 254 Nmm in group A and 331 ±176 N.mm in Group B (p=0.01). Mean pin extraction torque was 191±155 Nmm in Group A and 600 ± 214 Nmm inGroup B (p < 0.05), and in Group B it was higher than the corresponding insertion torque (p =0.001). The osteoconductivity of hydroxyapatite enabled bone remodelling and direct bony coverage of the pin surface and thus, mechanically improved the interface strength. The surface of the hydroxyapatite coating may also have been responsible for the improvement because it is rougher than the normal pin surface and may cause a higher initial mechanical stability. The higher fixation was also observed under unfavourable mechanical conditions. The benefits provided by the coating were more evident in the cancellous bone than in the cortical bone. In the standard pins, there was a progressive deterioration of the strength of fixation in both bone types, expressed by a pin extraction torque lower than pin insertion torque. In the hydroxyapatite-coated pins, an increase in pin fixation was observed. This increase was higher in the cortical bone than in the cancellous bone.

Bisphosphonates

Another innovative approach to enhance implant fixation is the use of either systemic or local administration of bisphosphonates. Bisphosphonates have a particular affinity for areas of increased bone turnover, in particular, around the fracture site. Bisphosphonates attracted to the hydroxyapatite in the bone will firstly inhibit bone resorption by being incorporated selectively into osteoclasts and, secondly, by interfering with the cells. Previous animal studies have shown that bisphosphonates can improve early fixation in both cortical and cancellous bone (9). Other studies have also demonstrated that alendronate inhibits bone resorption at the bone-screw interface thereby enhancing fixation (10). This active bone remodelling initiated by the alendronate around the implant permits good bone-screw fixation and the prevention of pin loosening or infection.

Conclusions

The achievement of stable fixation in osteoporotic bone is dependent on the assessment of bone quality using DXA prior to surgery and the use of augmentation techniques. This in turn will allow for early resumption of muscle function, joint mobilization and functional independence. We believe that fixation augmentation techniques dedicated to achieving adequate stability in low-energy trauma patients with osteoporosis should be considered a favourable option by the orthopaedic surgeon. The majority of implants currently in use were designed for normal bone, therefore, ignoring the needs of this special patient population. The biomechanical and biological problems inherent in this type of fixation dictate that implants should be designed specifically for use in osteoporotic bone. Secondly, our focus should be directed towards the improvement of bone strength. Porous bone responds positively to the injection of calcium phosphate cements and to the use of both bone grafts and calcium phosphate implants. Finally, we must consider improving the capacity of the implant to bind directly to the bone. This can be accomplished by coating the implant surface with osteoconductive materials such as hydroxyapatite. Therapy with bisphosphonates is another attractive augmentation concept, which warrants further investigation.

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