The effects of carpentry on heterotopic ossification and mobility in cervical arthroplasty: determination by computed tomography with a minimum 2-year follow-up

Clinical article

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Heterotopic ossification (HO) after cervical arthroplasty can limit the mobility of an artificial disc. In this study the authors used CT scanning to assess the formation of HO with the goal of investigating the correlation between the carpentry of arthroplasty, formation of HO, mobility, and clinical outcomes.


A retrospective review of medical records, radiological studies, and clinical evaluations was conducted for consecutive patients who underwent 1- or 2-level cervical arthroplasty with the Bryan disc. The patients underwent follow-up for more than 24 months. The formation of HO was assessed using CT scanning as the final determination. The perfectness of carpentry for each arthroplasty level was scrutinized using criteria composed of 2 parameters (postoperative shell kyphosis and inadequate endplate coverage). Levels were divided into the optimal carpentry group and the suboptimal carpentry group. Radiographic and clinical outcomes, including the visual analog scale and neck disability index, were compared between the groups.


A total of 107 levels of Bryan discs were placed in 75 patients (mean age 46.71 ± 9.94 years) and were analyzed. There was a male predominance of 68.0% (51 men), and the mean follow-up duration was 38.56 ± 9.66 months. Heterotopic ossification was identified in 60 levels (56.1%) by CT scanning. Most cases of HO were low grade and did not correlate with the limitation in the segmental motion of the arthroplasty device. There were no significant differences in terms of age, sex, and number of arthroplasty levels between the optimal and the suboptimal carpentry groups. However, the suboptimal carpentry group had significantly more high-grade HO (≥ Grade 2) than the optimal carpentry group (13 levels [12.1%] vs 7 levels [6.5%], p = 0.027). There were also more immobile (range of motion < 3°) artificial discs in the suboptimal carpentry group than the optimal carpentry group (11 levels [10.3%] vs 4 levels [3.7%], p = 0.010). The clinical outcomes (neck and arm visual analog scale scores and Neck Disability Index) in both groups were similarly good.


Shell kyphosis and inadequate endplate coverage have adverse effects on the formation of HO and segmental mobility after cervical arthroplasty with the Bryan artificial disc. Appropriate carpentry is the more important factor in determining the maintenance of segmental motion. Although the midterm clinical outcome remained similarly good regardless of HO, the carpentry of cervical arthroplasty should not be overlooked. Further studies are needed to clarify the etiology of HO.

Abbreviations used in this paper:HO = heterotopic ossification; NDI = Neck Disability Index; NSAID = nonsteroidal antiinflammatory drug; ROM = range of motion; VAS = visual analog scale.

Article Information

* Drs. Tsung-Hsi Tu and Jau-Ching Wu contributed equally to this work.

Address correspondence to: Chin-Chu Ko, M.D., Department of Neurosurgery, Neurological Institute, Taipei Veterans General Hospital, Room 509, 17F, No. 201, Shih-Pai Road, Sec. 2, Beitou, Taipei 11217, Taiwan. email:

Please include this information when citing this paper: published online March 30, 2012; DOI: 10.3171/2012.3.SPINE11436.

© AANS, except where prohibited by US copyright law.



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    Postoperative neutral lateral radiographs showing the measurements of the shell angles. A and B: An image without (A) and with (B) markers showing the presence of segmental kyphosis. Here, a kyphotic shell angle of 10° and formation of Grade 2 HO are seen. C and D: An image without (C) and with (D) markers showing an acceptable lordotic shell angle. A lordotic shell angle of 7° and no HO are seen.

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    Postoperative lateral radiographs showing measurements of the adequacy of endplate coverage. A and B: An image without (A) and with (B) markers showing inadequate endplate coverage. A discrepancy (gap size) of 2.5 mm and formation of Grade 1 HO (arrow) can be seen. C and D: An image without (C) and with (D) markers showing complete (nicely fitted) endplate coverage. No gap is seen between the posterior borders of the vertebral bodies and the shells.

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    Lateral neutral and dynamic radiographs showing 2-level cervical arthroplasty in the optimal and the suboptimal carpentry groups. A–C: Images showing optimal carpentry in neutral (A), flexion (B), and extension (C) obtained in one patient. The images show no HO formation and good segmental mobility (full ROM) at both levels. The sum score is as follows: sum score = 1 (no shell kyphosis [0 points], adequate endplate coverage [1 point]). There is no HO and normal mobility (ROM 8°). D–F: Images showing suboptimal carpentry in neutral (D), flexion (E), and extension (F) obtained in another patient. The sum score is as follows: sum score = 4 (shell kyphosis [1 point], inadequate endplate coverage [parallel lines], gap size > 2 mm [3 points]). There is Grade 3 HO (arrowhead) and limited mobility (ROM 3°).

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    Graph showing the VAS scores for arm and neck pain at each time point. There were no statistically significant differences between the 2 groups at all time points.

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    Graph showing the NDI. There were no statistically significant differences between the 2 groups at all time points.

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    Heterotopic ossification as seen in the literature. A: Image demonstrating advanced HO formation in an arthroplasty level with substantially inadequate endplate coverage. Reprinted with permission from Pickett et al: J Neurosurg Spine 4:98–105, 2006. B–D: Images also demonstrating severe HO in a level of arthroplasty with inadequate endplate coverage. Reprinted with permission from Parkinson et al: J Neurosurg Spine 2:377–380, 2005.



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