Editorial. Bisphosphonates do not impair spinal fusion

Abstract

O steOpOrOsis is defined as diminished bone mineral density and bone quality resulting in increased risk of fracture. This is a chronic condition that is underappreciated and undertreated, even after the occurrence of fragility fractures. 1,2 Osteoporosis is also very common in patients undergoing elective spine surgery, occurring in 20% or more of patients, and rates are often higher in patients undergoing revision or deformity surgery. 3 Two classes of medications are used to treat osteoporosis: antiresorptive and anabolic. Antiresorptive medications include bisphosphonates, denosumab, calcitonin, and estrogens and selective estrogen receptor modulators. Bisphosphonates are the most commonly used drugs for treating osteoporosis because they are inexpensive (generic options are available), usually well tolerated, and can be given orally. However, their mechanism of action (inhibition of osteoclasts) and their theoretical negative effect on bone healing, which may follow an uncoupling of balanced osteoclastic and osteoblastic activities, has caused concern among spine surgeons that bisphosphonates will interfere with bone healing after fusion. Bisphosphonates inhibit a key enzyme of the mevalonate pathway that reduces osteoclastic bone resorption ability and induces osteoclastic apoptosis. 4 These mechanisms of action result in a rapid decrease in bone turnover along with increased bone mineral density and lower fracture risk. In fracture models in animals treated with bisphosphonates, bone remodeling of the callus is slowed but mechanical strength is maintained as a larger callus is formed in treated than control animals. A similar phenomenon likely occurs during spine fusion. Although results in animal studies are conflicting with regard to whether spine fusion is affected by bisphosphonates, 5,6 bone healing after fractures in humans, including spinal fractures, does not appear to be affected by the use of bisphosphonates either before or after fracture. 7,8 Bisphosphonates are analogs of pyrophosphate and Disclosures Dr. Anderson reports direct stock ownership in Titan Spine; being a consultant for Medtronic, Amgen, and Radius Medical; and receiving royalties from Regeneration Technologies.