Heterotopic ossification in cervical disc arthroplasty: Is it clinically relevant?

Abstract

Study design:Retrospective cohort study.

Objective:To analyze the presence and clinical relevance of heterotopic ossification (HO) at 3 years mean follow-up.

Methods:Thirty patients suffering from cervical radiculopathy and / or myelopathy treated with anterior disc replacement (ADR) were studied. HO was classified using the McAfee grading system. Range of motion was measured from flexion and extension x-rays. Short-form 36 and neck disability index (NDI) assessed functional outcome.

Results:Forty-five prostheses were implanted in 30 patients with cervical radiculopathy and / or myelopathy, mean age 40.9 years. Nineteen patients received 1 level and 11 patients received multilevel disc replacement. The incidence rate of HO was 42.2% (19 levels). Segmental range of motion was ≥ 3 ° in 93.8% of patients with HO. There was no significant difference in functional scores between those who did and those who did not develop HO. Males tended to develop HO more frequently than females, though this was not statistically significant. The indication for surgery (soft disc hernia or spondylosis) was not associated with the formation of HO.

Conclusions:Functional improvement is maintained despite the presence of HO following cervical disc arthroplasty. Indications for arthroplasty should not be halted by the risk of HO.

References

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Editorial staff perspectives

This is a CoE III prognostic study.

Disc replacement offers a new interventional option and long-term data on the potential complications to artificial disc replacement compared with fusion are very much needed. This prospective study's contribution to the literature regarding heterotopic ossification (HO) following cervical arthroplasty is appreciated. As the authors note, the causes of HO in disc arthroplasty remain unclear and are probably multifactorial.

HO frequency:

The authors report a HO rate of 42% following artificial disc replacement in 32 patients, a higher overall rate than is reported in most other studies. HO rates in the May 2008 EBSS Special Edition on cervical arthroplasty were summarized as follows: There were no HO cases reported in two RCTs, with follow-up of 24 months in one study and other being a preliminary report with most patients having only 12-months follow-up. HO was, however, common in two case series with one reporting an overall 18% rate or 7% cases when restricted to grades III or IV. The other series of patients with 1, 2, or 3 level disease reported that 8% of segments had grade I, 39% of segments had grade II, 10% had HO leading to restricted movement and 9% experienced spontaneous fusion.

There are several factors which may partially explain discrepancies in HO rates across studies, aside from differences in length of follow-up. It is possible that increased attention to radiographic detail may go hand in hand with increased reporting. Grading the severity of HO is not an exact science. There is some subjectivity which may translate into difficulty in distinguishing between adjacent grades (eg, between grades 2 and 3) and overlap in classification. The grading of HO has so far not been subjected to inter- and intra-observer reliability evaluation and is not known to correlate with any health-related quality of life (HrQoL) outcomes measures.

Study specifications and protocols:

Use of structured measures and protocols in a prospective study, which are consistently applied to all patients, is important to decrease study bias. While the authors suggest that structured protocols were used, no details were provided. Reporting detail about protocols assists in determining the extent to which various factors may or may not have influenced the results. For studies of HO, important protocol details should include whether or not bone waxing was done at the osteotomy site as well as specifics for radiographic measurements and use of antiinflammatory medications. Description of post-operative rehabilitation is also important. It is unclear whether early initiation of range of motion creates an inflammatory response which contributes to HO formation or if it facilitates functional outcome.

Strengths:

The use of validated outcomes measures (NDI and SF-36) is commendable as is the authors' acknowledgment of small sample size and a relatively low follow-up rate (77%) as study limitations.