Free radical scavengers are more effective than indomethacin in the prevention of experimentally induced heterotopic ossification

Allopurinol and oxypurinol promote osteoblast differentiation and increase bone formation

Allopurinol and its active metabolite, oxypurinol are widely used in the treatment of gout and hyperuricemia. They inhibit xanthine oxidase (XO) an enzyme in the purine degradation pathway that converts xanthine to uric acid. This investigation examined the effect of allopurinol and oxypurinol on bone formation, cell number and viability, gene expression and enzyme activity in differentiating and mature, bone-forming osteoblasts. Although mRNA expression remained relatively constant, XO activity decreased over time with mature osteoblasts displaying reduced levels of uric acid (20% decrease). Treatment with allopurinol and oxypurinol (0.1-1 µM) reduced XO activity by up to 30%. At these concentrations, allopurinol and oxypurinol increased bone formation by osteoblasts ~4-fold and ~3-fold, respectively. Cell number and viability were unaffected. Both drugs increased tissue non-specific alkaline phosphatase (TNAP) activity up to 65%. Osteocalcin and TNAP mRNA expression was increased, 5-fold and 2-fold, respectively. Expression of NPP1, the enzyme responsible for generating the mineralisation inhibitor, pyrophosphate, was decreased 5-fold. Col1α1 mRNA expression and soluble collagen levels were unchanged. Osteoclast formation and resorptive activity were not affected by treatment with allopurinol or oxypurinol. Our data suggest that inhibition of XO activity promotes osteoblast differentiation, leading to increased bone formation in vitro.

Risk factors for heterotopic ossification in patients with spinal cord injury: a case-control study of 264 patients

STUDY DESIGN

Case-control study.

OBJECTIVE

We designed a case-control study to analyze the risk factors associated with the development of heterotopic ossification (HO) in patients with traumatic spinal cord injury.

SUMMARY OF BACKGROUND DATA

Patients with spinal cord injury have a high risk of developing HO, although the exact etiopathogenesis is still unknown. Several factors are known to be potential risk factors. However, we are not aware of any large clinical studies evaluating the risk factors for HO.

METHODS

Patients who were treated for a traumatic spinal cord injury in our hospital, and who subsequently developed HO, were identified by querying the electronic database at our hospital from 2002 to 2010. One hundred thirty-two patients and 132 controls were included. Our primary outcome measures were the risk of developing HO according to whether the patient had experienced a complete spinal cord lesion according to American Spinal Injury Association Impairment Scale; tetraplegia or paraplegia; cervical, thoracic, or lumbar injury; severe chest trauma; and the time interval between injury and surgery. Secondary risk factors explored were patient age; sex; presence and number of comorbidities; length of hospital and intensive care unit stay; associated traumatic injuries; presence of spasticity, pressure ulcers, deep venous thrombosis, and urinary tract infection; and pulmonary complications, such as pneumonia and necessity of tracheostomy.

RESULTS

Patients with associated spasticity and thoracic trauma, complete lesion, pneumonia, presence of tracheostomy, and urinary tract infection had a higher risk of developing HO.

CONCLUSION

Adequate management of potential risk factors could help reduce the overall incidence of HO and outcome in patients with traumatic spinal cord injury.

Pharmacological treatment of heterotopic ossification following hip surgery: an update

Heterotopic ossification is a frequent complication following total hip arthroplasty and acetabular surgery. Formation of ectopic lamellar bone in tissues that exhibit no potential for ossification can lead to associated pain and decrease in function. Prophylaxis and treatment protocols aim to reduce the incidence, by both the use of nonsteroidal anti-inflammatory drug (NSAID) regimens and localized radiotherapy. New therapeutic modalities including bone morphogenetic proteins (BMP) inhibitors like Noggin, BMP type 1 receptor inhibition, nuclear retinoic acid receptor-gamma agonists (RAR-γ), and free radical scavengers are currently under investigation.

Heterotopic ossification following traumatic brain injury and spinal cord injury

Heterotopic ossification associated with neurologic injury, or neurogenic heterotopic ossification, tends to form at major synovial joints surrounded by spastic muscles. It is commonly associated with traumatic brain or spinal cord injury and with other causes of upper motor neuron lesions. Heterotopic ossification can result in a variety of complications, including nerve impingement, joint ankylosis, complex regional pain syndrome, osteoporosis, and soft-tissue infection. The associated decline in range of motion may greatly limit activities of daily living, such as positioning and transferring and maintenance of hygiene, thereby adversely affecting quality of life. Management of heterotopic ossification is aimed at limiting its progression and maximizing function of the affected joint. Nonsurgical treatment is appropriate for early heterotopic ossification; however, surgical excision should be considered in cases of joint ankylosis or significantly decreased range of motion before complications arise. Patient selection, timing of excision, and postoperative prophylaxis are important components of proper management.

Free radical scavengers versus methylprednisolone in the prevention of experimentally induced heterotopic ossification

The etiology of heterotopic ossification (HO) is still obscure, it is difficult to devise an effective preventive or therapeutic approach. The options for the prevention of HO are still limited. The prophylactic effect of nonsteroidal anti-inflammatory drugs (NSAIDs) is insufficient. Moreover, NSAIDs increase the risk of nonunion and loosening in patients with multiple joint injuries. The present experimental study was designed to compare methylprednisolone with free radical scavengers for the prevention of HO. The model of Michelsson et al. was used to induce HO in the hind legs of 30 female New Zealand albino rabbits, weighing approx. 4 kg. The animals were randomized into three groups of 10 animals each, and received daily either placebo, a free radical scavenger cocktail [allopurinol and N-acetylcysteine (A/A)], or methylprednisolone in a randomized, double-blind fashion. Every four days, X-rays were obtained to measure the thickness and the length of new bone formation at the thigh. A statistically significant difference in thickness and length of newly formed bone was found between the three groups. In the placebo group HO increased from day 16 toward a medium length of 6 mm and a median thickness of 1.5 mm. In the A/A group, no signs of HO were found. In the methylprednisolone group, only one animal presented minor HO from day 32. Both free radical scavengers and methylprednisolone were found to inhibit HO, and may be considered effective measures for the prevention of heterotopic bone formation. However, it could not be demonstrated which of the two had the strongest inhibitory effect.

Skeletal metamorphosis in fibrodysplasia ossificans progressiva (FOP)

Metamorphosis, the transformation of one normal tissue or organ system into another, is a biological process rarely studied in higher vertebrates or mammals, but exemplified pathologically by the extremely disabling autosomal dominant disorder fibrodysplasia ossificans progressiva (FOP). The recurrent single nucleotide missense mutation in the gene encoding activin receptor IA/activin-like kinase-2 (ACVR1/ALK2), a bone morphogenetic protein type I receptor that causes skeletal metamorphosis in all classically affected individuals worldwide, is the first identified human metamorphogene. Physiological studies of this metamorphogene are beginning to provide deep insight into a highly conserved signaling pathway that regulates tissue stability following morphogenesis, and that when damaged at a highly specific locus (c.617G > A; R206H), and triggered by an inflammatory stimulus permits the renegade metamorphosis of normal functioning connective tissue into a highly ramified skeleton of heterotopic bone. A comprehensive understanding of the process of skeletal metamorphosis, as revealed by the rare condition FOP, will lead to the development of more effective treatments for FOP and, possibly, for more common disorders of skeletal metamorphosis.