Elsevier

The Spine Journal

Volume 13, Issue 4, April 2013, Pages 408-420
The Spine Journal

Clinical Study
An analysis of heterotopic ossification in cervical disc arthroplasty: a novel morphologic classification of an ossified mass

https://doi.org/10.1016/j.spinee.2012.11.048Get rights and content

Abstract

Background context

Although the precise cause of heterotopic ossification (HO) remains unclear, it is certain that it increases with time. The reason why the reported occurrence rate has been given as a wider range is that there were no clinical reports that have addressed the occurrence rate based on the morphology and position.

Purpose

The aim of this retrospective study was to determine whether radiological parameters had an influence on the formation of HO and to compare the results after cervical arthroplasty using Bryan (Medtronic Sofamor Danek, Memphis, TN, USA), PCM (Cervitech, Rockaway, NJ, USA), and Prestige LP (Medtronic Sofamor Danek) implants.

Study design/setting

A retrospective study.

Patient sample

Eighty-one patients were included.

Outcome measurement

The occurrence of HO was determined at the four corners of the disc space according to the McAfee classification system. Heterotopic ossifications were classified into Type 1, Type 2, and Type 3 HOs (end plate, traction spur, and teardrop types) based on their morphologic features. The presence of preoperative ossifications, sex, type of device, operated level, hybrid implantation, depth behind the prosthesis, cervical lordotic angle, and segmental angle between the footplates of the prosthesis were assessed as predictors in terms of location and morphologic features.

Methods

Eighty-one patients after 95 cervical arthroplasties using the Bryan (35 segments), PCM (30 segments), and Prestige LP implants (30 segments) underwent postoperative radiographs or three-dimensional computed tomography with a mean follow-up period of 46, 39, and 30 months, respectively, after the operation.

Results

The overall incidence of all cases and one-level subgroup were 64.2% and 60.3%, respectively. According to the types of device, the incidence of HO was 49% (Bryan), 80% (PCM), and 60% (Prestige). Type 1 HO (62.1%) was found only in the posterosuperior disc space. Type 2 HO (13.7%) was primarily detected in the anterosuperior disc space rather than the posterosuperior disc space (3.2%). Type 3 HO (4.2%) developed only in the anterior disc space. In the anterior disc space, the incidence of Type 2 and Type 3 HOs was highest in the PCM group. In the posterior disc space, the Bryan group showed a lower proportion in the high McAfee class than the other device groups. The occurrence of Type 1 HO in the posterosuperior disc space was significantly related with the presence of preoperative ossification (p=.030), ossification in the ligamentum nuchae (p=.027), male sex (p=.042), and PCM device (p=.012). A well-fitting (p<.002) and less lordotic segmental angle (p<.015) were correlated with Type 1 HO. Implantation in the upper cervical level (p=.016) and hybrid implantation with cage (p=.033) or artificial disc (p=.048) on the upper adjacent level were significant risk factors for anterior Type 2 HO. Cervical lordotic angle at 1 month after surgery had a significant connection with the occurrence of anterior Type 2 HO in both groups of all cases (p=.032) and one-level subgroup (p=.000).

Conclusions

Type 1 HO developed mostly in the posterior disc space. Type 2 HO was the dominant type in the anterior disc space. Type 3 HO developed only in the anterior disc space. It is certain that both Type 1 and Type 2 HOs are related to biomechanical stresses (compressive force for Type 1 HO and traction force for Type 2 HO). It is suggested that a cervical arthroplasty should be selected in terms of the implant level, hybrid conditions on the upper adjacent segment, disc design vulnerable to the pseudotranslation, the presence of preoperative ossification, and fitting implants to end plates to reduce the development of HO.

Introduction

Evidence & Methods

HO following cervical disc arthroplasty can potentially impact mobility and performance of the device. In this retrospective radiographic review, the authors characterize HO in their patients who underwent surgery using three different devices.

The authors found that HO was most common when the PCM implant was used (80%), followed by Prestige (60%), and Bryan (49%). They also characterize the type of ossification noted.

The design of the study is strictly observational. The study design does not allow conclusion that a particular device is responsible for more or less HO given that patient and surgeon-specific (technical) confounders may well be present. Nonetheless, the large differences in observed rates of HO suggest that high-quality, controlled, comparative trials are needed and would not require very large numbers of subjects to detect this magnitude of effect.

The Editors

Cervical total disc replacement (TDR) is known to be a good surgical option for cervical disc disease and have satisfactory radiological and clinical outcomes compared with anterior cervical discectomy and fusion. However, heterotopic ossification (HO) after TDR, one of the midterm complications, is an embarrassing phenomenon. Many studies indicated that HO after TDR did not have negative influence on the clinical outcome, but it is important to note that HO impairs the range of motion in index segments [1], [2], [3], [4], [5], [6], [7], [8]. Several clinical studies have focused on the incidence and risk factors of HO. The occurrence rate of HO varied according to disc prostheses: 21% to 76% at 2 years for Bryan (Medtronic Sofamor Danek, Memphis, TN, USA) [1], [2], [3], [4], 71% to 79% at 2 years for Prodisc-C (Synthes Inc., Paoli, PA, USA) [2], [5], [6], 14% at 14 months for Prestige LP (Medtronic Sofamor Danek, Memphis, TN, USA) [7], and 0.4% at 1 year for PCM (Cervitech, Rockaway, NJ, USA) [8]. It has recently been reported that the incidence and grade of HO increased as time passed and spontaneous fusion must be anticipated during long-term follow-up [6]. Male sex, older age, and multisegmental operation were found to be the risk factors for the development of HO [1]. Factors suggested to contribute to the HO included bone dust, wear debris of the metal on the polyethylene component, the keeling procedure in the Prodisc-C prosthesis, and stress at the interface between the end plate and prosthesis [1], [2], [9].

Heterotopic ossifications in TDR are quite different from HOs in hip and knee joint surgery in terms of location, absence of preceding inflammatory symptoms or signs, and the temporal pattern of growth. Heterotopic ossification in TDR looks more likely to fusion mass after interbody fusion or osteophyte in the aging spine rather than usual heterotopic ossification. Therefore, the authors hypothesized that mechanical factors might play a key role in the development of HO in TDR and the configuration of HO would be different according to the mechanical environment. Recently, we developed a novel morphologic classification of HO in cervical TDR. The purpose of this study was to understand the pathogenetic mechanism and risk factors for HO in cervical TDR based on the morphologic subtype and location.

Section snippets

Materials and methods

Between August 2004 and November 2009, 83 consecutive patients underwent cervical disc arthroplasty using Bryan, PCM, or Prestige LP. All patients received follow-up care for at least 20 months postoperatively (Bryan group: 46 months, PCM group: 39 months, and Prestige group: 30 months). The average follow-up period was 38 months (Table 1). Three surgeons performed arthroplasties for 95 segments in 81 patients (67, one level and 14, two levels). There were 35 Bryan discs, 30 PCM discs, and 30

Overall incidence of HO

The overall incidences of all types of HO were 64.2% and 60.3% for One-level subgroup. According to the location, the occurrence of HO in the anteroinferior (11.6%, all Type 2) and posteroinferior disc spaces (40%, all Type 1) was much lower than that in the anterosuperior and posterosuperior disc spaces. The incidences of HO in the anterior and posterior disc spaces were 17.9% and 62.1%, respectively (9.5% and 56.6% of one-level subgroup) (Fig. 5).

Incidence of HO according to morphologic classification

Type 1 HO was found only in the

Discussion

There are some differences between HO in hip and knee joint surgery and HO in cervical TDR. For the basic pathogenesis of HO, it is known that primitive cells of mesenchymal origin, which are present in the connective tissue septa within the muscle, are transformed into osteogenic cells [13]. However, most of HOs in cervical TDR are located in the vicinity of the footplate of disc. Moreover, HO in cervical TDR is not related to the presence of muscular tissue. The inflammatory symptoms such as

Conclusion

The incidences of HO in the anterior and posterior disc spaces were 17.9% and 62.1% of all cases, respectively. Type 1 developed mostly in the posterior disc space. Type 2 was the dominant type in the anterior disc space. Type 3 (4.2% of all cases) developed only in the anterior disc space. It is certain that both Type 1 and Type 2 HOs are related to biomechanical stresses and have different pathogenetic mechanisms (compressive force for Type 1 and traction force for Type 2). It is suggested

Acknowledgments

This study was financially supported by a grant from Inje University, 2010. The authors thank M-JK for the collection of related data.

References (21)

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FDA device/drug status: Approved (Bryan disc, Prestige LP disc, PCM disc).

Author disclosures: YJJ: Nothing to disclose. SBP: Nothing to disclose. MJK: Nothing to disclose. K-JK: Nothing to disclose. H-JK: Nothing to disclose.

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