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Ross K. Leighton, M.D., FRCSC
Halifax, NS As a conservative estimate, surgeons perform 500,000 bone-grafting procedures per year in the United States and approximately 50,000 procedures per year in Canada. Musculoskeletal tissue grafts often serve both a mechanical and biological function. The choices of graft material, of course, are large including that of cortical bone, cancellous bone, processed allograft bone, and synthetic materials.
The substitutes can be either osteoconductive or osteoinductive. Osteoinduction refers to the recruitment of host mesenchymal cells from the surrounding soft tissue or bone that then differentiate into bone-forming cells. An example of an osteoinductive material is bone morphogenic protein.
Osteoconduction is the process of ingrowth of blood vessels, perivascular tissue, and osteoprogenitor cells from the host bed into the graft structure. The graft serves as a scaffold for the ingrowth of new bone (i.e. Alpha BSM®).
The choices of osteoconductive materials include that of porous, corraline ceramics-tricalcium phosphate implants, collagen, and calcium phosphate cements. Most of these materials attempt to mimic the mineral phase of bone. The ideal bone graft substitute would, of course, be osteogenic, biocompatible, bioabsorbable, provide structural support, be easy to use clinically, and, of course, be cost effective.
Alpha BSM®, which is an osteoconductive material, does provide a lot of the above-stated ideal properties. It is an injectable, endothermically setting material. It has chemical properties similar to bone, which allows it to be biocompatible, as well as biodegradable.
Periarticular fractures are common in indirect coronal and/or direct axial compressive forces. The resulting fracture pattern (i.e., a tibial plateau fracture or Pilon fracture) is related to the age of the patient and their overall bone quality. As the patient ages, the fracture pattern is usually a depression type without associated ligamentous injury. Although there remains some controversy concerning the operative indications for treatment of this type of injury, the AO guidelines advocate some type of anatomic reduction, re-establishment of the articular surface along with subchondral bone grafting to support the articular cartilage and stable fixation.
The current accepted treatment for this type of injury is stabilization of the subchondral void with cancellous bone graft retrieved from an iliac crest donor site. This procedure involves making a separate incision over the crest to obtain the graft, which can cause morbidity, such as pain, nerve injury, arterial injury, cosmetic deformity and infection. There is currently no literature that documents the stability or compressive strength of this type of fracture fixation, although it is the gold standard for treatment. In recent years, new bone alternatives have become commercially available to act as “substitute” bone graft. These consist of various forms of ceramics, such as tricalcium phosphate and hydroxyapatite.
The bone graft discussed here is Alpha BSM®, which is an injectable, endothermically setting material that has chemical properties similar to bone. There is very little in the literature that documents the compressive strength of these materials and whether or not they function as a support structure within the cancellous bone. In the study performed at Dalhousie University by Dr. Andrew Trenholm et al, 10 matched pairs of tibias were set up for analysis using Alpha BSM® versus cancellous bone graft. Each individual test began by positioning the potted tibia in an adjustable jig and 38 mm modified spherical indenter was designed to ensure that the contacting surface between the indenter and the tibial plateau occurred only over the defect.
Displacement curves were generated for each pair of tibias, and from these curves, two parameters were defined to quantify the strength of the construct defect displacement at 1000 Newtons and also to evaluate initial stiffness. In the results of this in-vitro study, the Alpha BSM® tibias were stronger than the cancellous bone-repaired tibias. Using the paired student T test, statistical significant differences (P less than 0.05) were found in displacement at 1000 Newtons and initial stiffness between the repaired tibial construct. The stiffness was greater, and the displacement at 1000 Newtons was significantly less for the Alpha BSM® tibias.
The present study, briefly described above, demonstrates that an Alpha BSM®-repaired construct has superior initial compressive strength and initial stiffness properties in comparison to a cancellous bone-repaired construct. For example, a split depression fracture of the lateral tibial plateau can create a dilemma for the treating orthopaedic surgeon. The results with these fractures are better when anatomic reduction of the joint line is obtained and early range of motion is instituted. There is, therefore, a conflict between early motion to promote articular cartilage healing and maintenance of the reduction to prevent future deformity. Our findings indicate that Alpha BSM® is significantly stronger in compression than cancellous bone graft. Other potential clinical advantages of Alpha BSM® are:
- Potentially, to allow for earlier weightbearing of these fractures to promote range of motion and joint health without the fear of losing the reduction.
- The lack of morbidity associated with its use when compared to traditional autogenous bone graft.
- No risk of viral transmission as exists with allograft bone.
With periarticular depression fractures occurring with higher frequency in the elderly (who are often unable to touch toe or non-weightbear secondary to decreased upper body strength), a bone graft substitute with increased compressive strength is of potential benefit.
There has been an influx of bone graft substitutes in the orthopaedic realm over the last few years but most have not been thoroughly studied nor their clinical roles clearly defined. In the clinical setting of a periarticular fracture with joint depression, it would be advantageous to prove that our chosen bone graft substitute is able to meet the requirements of biocompatibility and provide structural stability to the elevated joint line.
There are presently a number of clinical evaluations in progress:
- An FDA study on tibial plateau fractures has almost reached its completion.
- Two studies that are presently under initiation by the Canadian Orthopaedic Trauma Study Group are looking prospectively at os calcis fractures and distal radial fractures.
These fracture groups are all periarticular and all require support in order to prevent collapse of the joint fragments in the peri-operative or peri-reduction time period.
It should be clearly noted that Alpha BSM® has not been shown by itself to provide enough support to any of these fracture patterns. However, Alpha BSM®, when used in conjunction with traditional open reduction internal fixation techniques may indeed improve the support of the subchondral bone and reduce the collapse that we would normally see in these types of fractures. The lab data (reported at the AAOS 2001; Trenholm et al) is statistically significant in that Alpha BSM®, when placed in those fractures that undergo compression, provides better structural support for the articular surface than the more commonly used cancellous bone.
Alpha BSM® does not have any osteoinductive capabilities but is osteoconductive and, therefore, is best utilized in a contained defect within cancellous bone.
However, having said this, the results of the presently ongoing randomized prospective clinical trials will help to define and delineate the role of this material in the care of the orthopaedic patient. Of note, Alpha BSM® has been approved for clinical use in Canada over the last 2 _ years and has recently received FDA approval in 2001.
References
- Muller M.E., Allgower M., Schneider R., Willenegger H.,: Manual of Internal Fixation, Third Edition, Springer-Verlag 1995
- Kurz L.T., Garfin S.R., Booth Jr. RE: Harvesting Autogenous Iliac Bone Grafts A Review of Complications and Techniques. Spine Volume 14 Number 12, 1324-1331, 1989.
- Lee D.D., Tofighi A, Aiolova M, Chakravarthy P, Catalona A, Majahad A, Knaack D: @-BSM : A Biomimetic Bone Substitute and Drug Delivery Vehicle. Clinical Orthopaedics And Related Research Number 367s , s396-s405, 1999
- Beris A.E., Soucacos P.N., Glisson R.R., Seabar A.V., Urbaniak J.R.: Load tolerance of tibial plateau depressions reinforced with a cluster of K-wires. Bulletin Hospital for Joint Diseases 55:12-15, 1996.
- Stuart M.J., Beachy A.M., Grabowski J.J., An K.N., Kaufman K.R.: Biomechanical evaluation of a proximal tibial opening-wedge osteotomy plate. Am J Knee Surg 12:148-154, 1999.
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