Keys To Optimal Selection Of Orthobiologics
Advantages of osteoconductive substitutes are the fact that they are readily available, relatively inexpensive and serve a role in void filling. There is also an absence of donor site morbidity and viral transmission. The main disadvantage of these calcium compounds is the limited or lack of biological healing potential.2 Examples of osteoconductive orthobiologics include demineralized bone matrix (DBM), which is both osteoconductive and osteoinductive, calcium phosphate, and calcium sulfate.
Calcium sulfate is available in many forms. These sulfates are available as hard pellets and injectable fluids that harden in vivo. One can load calcium sulfate with antibiotic to help treat osteomyelitis. In a prospective study of 25 patients requiring debridement of long bone infection, McKee and colleagues found that infection was eradicated in 23 patients.3 Moreover, study authors noted unions in 14 out of 16 previous non-union patients with the use of tobramycin-impregnated calcium sulfate. Substantiating the claim that sulfate compounds are highly effective bone defect fillers, Gitelis and coworkers were able to attain healing in 21 out of 23 patients with bone defects, using calcium sulfate either in combination with demineralized bone matrix or in isolation.4 Demineralized bone matrix and calcium sulfate also remain the most rapidly absorbed bone substitutes.
Calcium phosphate substitutes are calcium salt compounds made up of calcium ions and organophosphates. Most of these compounds are available as a mixture of calcium salts and few are available as pure formulations.2 Compounds used in these bone grafts consist of mono-, di-, tri- and tetracalcium phosphates. Surgeons frequently use hydroxyapatite in its alpha and beta forms. Tricalcium phosphate remains one of the most commonly used osteoconductive substrates.5 Cements are popular because they are effective in filling bone defects and provide structural resistance to compression. However, the risks of site migration and peripheral tissue damage remain concerns.5
In general, unique advantages of calcium phosphates include osteointegration, slow biodegradation and compressive strength. In cases in which osteoinduction is also desired, surgeons have the option of using composite grafts that combine osteoinductive factors with a calcium phosphate matrix. These compounds provide mechanical support in acute settings but they lack the biological ability to facilitate healing in chronic defects.5 Some of these grafts reportedly have up to ten times the compressive strength of cancellous bone. However, Roberts and Rosenbaum point out that these assertions do not take into account tensile and shear forces, which, along with compression, occur in vivo.2
There is no hard evidence that suggests osteoconductive orthobiologics reliably and reproducibly facilitate bone defect healing in humans.5 However, a number of researchers have reported clinical success. In a prospective cohort study of 32 patients with 36 joint depression calcaneal fractures, Schildhauer and colleagues found that calcium phosphate cement in conjunction with standard open reduction internal fixation allowed patients to bear weight as early as three weeks.6