Heterotopic Ossification Following Cervical Total Disc Replacement: Iatrogenic or Constitutional?

OBJECTIVE To elucidate etiological factors of heterotopic ossification (HO) by evaluating retrospectively if HO is a unique finding following cervical total disc replacement (CTDR) or a finding observable following an anterior cervical interbody fusion (ACIF). METHODS The authors had selected 87 patients who underwent anterior cervical surgery (TDR or ACIF), and could be followed up more than 24 months. A cervical TDR was performed using a Bryan disc or a ProDisc-C and an ACIF using a stand-alone cage or fibular allograft with a plate and screws system. The presence of HO was determined by observing plain radiography at the last follow up. The relation between HO occurrence and specific preoperative radio-logical findings (osteophyte and calcification of posterior longitudinal ligament (PLL)) at the index level was investigated. RESULTS Cervical TDR was performed in 40 patients (43 levels) and ACIF in 47 patients (54 levels). At the final radiographs, HO was demonstrated at 27 levels (TDR-Bryan; 8/18, TDR-Prodisc-C; 12/25, ACIF-cage alone; 7/29, and ACIF-plate screw; 0/25). Mean ROM at the last follow-up of each TDR subgroup were 7.8±4.7° in Bryan, 3.89±1.77° in Prodisc-C, and it did not correlated with the incidence of HO. Fusion status of ACIF groups was observed as 2 case of grade 1, 6 of grade 2, and 21 of grade 3 in cage alone subgroup, and no case of grade 1, 4 of grade 2, and 21 of grade 3 in plate screw subgroup. Fusion status in ACIF-cage alone subgroup was significantly related to the HO incidence. The preoperative osteophyte at the operated level observed in 27 levels, and HO was demonstrated in 12 levels (TDR-Bryan; 3/5, TDR-Prodisc-C; 2/3, ACIF-cage alone; 7/11, and ACIF-plate screw; 0/8). Preoperative PLL calcification at the operated level was observed 22 levels, and HO was defined at 14 levels (TDR-Bryan; 5/5, TDR-Prodisc-C; 4/5, ACIF-cage alone; 5/7, and ACIF-plate screw; 0/5). The evidence of preoperative osteophyte and PLL calcification showed statistically significant relations to the occurrence of HO. CONCLUSION HO was observed in both TDR and ACIF groups. HO was more frequently occurred in TDR group regardless of prosthesis type. In ACIF group, only cage alone subgroup showed HO, with relation to fusion status. Preoperative calcification of longitudinal ligaments and osteophyte were strongly related to the occurrence of HO.