INTRODUCTION

Ossification of the posterior longitudinal ligament (OPLL) as a cause of cord compression was first reported by Key in 1838 (1), and by Tsukimoto in Japan in 1960 (2). It was long thought to be a disease of the Japanese (in whom the incidence is 2.2 % in asymptomatic patients and 27 % in the presence of myelopathy). Improved imaging modalities and recognition of the disease have shown a 20 to 23 % incidence the Caucasian races in the presence of myelopathy (3,4). Recent reports have shown that non-Japanese Asians too may have an incidence close to that of the Japanese. However, there are no proper epidemiological studies to prove or refute this statement.

Clinically the patients present with symptoms of radiculomyelopathy, similar to those of spondylosis. One distinguishing feature, found in the author’s series was the singular absence of brachial neuralgia as a primary symptom. The latter was observed only in patients in whom a disc prolapse was the compressive element. Trauma aggravates symptoms in a patient with cervical OPLL. Relatively minor injuries can also precipitate severe symptoms and signs in an otherwise asymptomatic patient.

RADIOLOGY

Plain lateral radiographs of the cervical spine show OPLL as a dense band posterior to and extending across vertebral bodies. However, we found a false negative rate of In their prospective study of the cervical spine of 240 patients, Jayakumar et al (10) found evidence of OPLL in only 13 or 4.8%. The cause of the poor x-ray visibili¬ty of OPLL in patients of South Asia is not known. A similar low incidence of intracranial calcification, normal and pathological, has been reported in Indian patients and has been thought to be related to the type of nutri¬tion (26). This could account for the low incidence of OPLL reported from this part of the world. While plain x-rays and tomograms can be used to screen patients for OPLL, MRI and CT scans are essential to study the ossification.

Classification systems have been devised depending on the configuration of the OPLL on radiographs and sagittal MR scans. The most common system used is that which divides the OPLL into Continuous, Segmental, Mixed and Circumscribed types. Using axial CT scans OPLL has been classified into different types depending on shape (17). These systems do not help in the planning of surgery. A more useful system is where the OPLL is divided into “Localised” and “Extensive” types on the sagittal plane. The axial sections of the MR and / or CT scans may be used to study whether the OPLL is central and symmetrical , or, eccentric and asymmetrical in its configuration. These will influence decisions regarding approach and the technique of excision of the OPLL.

SURGICAL APPROACH

Surgical options for cervical OPLL include anterior and posterior decompres¬sive procedures. Which of these gives better results is still controversial (3,7,21,22). While anterior decompression and excision of the OPLL is preferred as it directly deals with the compressive pathology, a posterior decompressive procedure remains a popular choice when the cord is compressed for more than three vertebral segments . Baba et al (21) reporting on 85 patients who had undergone anterior decompression and fusion for cervical myeloradiculopathy due to OPLL found the anterior approach satisfactory only when done in upto two vertebral levels. For more extensive involvement they recommend posterior decompressive surgery. This is in contrast to the conclusions drawn by Epstein (3,22), who prefers the anterior approach . In her opinion, those patients who were in the worst pre-operative grade and underwent a corpectomy and fusion had the best recovery, while those who underwent a posterior procedure had the worst outcomes. We have found both anterior and posterior procedures to give good outcomes on followup. However, patients who undergo anterior procedures experience improvement early in the post-operative period , unlike patients who undergo posterior procedures. We recommend anterior approaches when the compressive element is three vertebral levels or less; and posterior procedures when the compressive element extends over three or more vertebral segments, or behind the body of the axis.

ANTERIOR SURGICAL PROCEDURES

The choice of surgical procedure is dependant on the patient’s clinical picture and the radiological findings. MR scans are the most useful for the planning of the surgical procedure. As the OPLL causes cord pressure from anterior, an anterior surgical procedures is ideal. However, technical difficulties related to exposure, resection of the OPLL, and, reconstruction and stabilisation of the vertebral column often preclude an anterior approach. When the compressive element extends for four vertebral segments or more, there is a preference for a posterior procedure as the condition resembles a cervical canal stenosis. However, in the presence of a pre-existing kyphosis, an anterior procedure is preferred. In practical terms therefore, the anterior approach is best performed when the cord compression extends for three vertebral segments or less, and in a kyphotic spine. A median corpectomy or an anterior segmental decompression are the surgical procedures available.

MEDIAN CORPECTOMY

SURGICAL TECHNIQUE
Shifting a patient with a cervical spine pathology to the operat¬ing table should be done with utmost care , and should be under the direct supervision of the surgical team. While ideally fibreoptic intubation may be practised, it is not essential as long as it is ensured that there are no vigorous neck movements during intuba¬tion. The endotracheal tube is preferably taken to the right side of the oral cavity. This takes the curve of the larynx to the left which helps retraction of the trachea during surgery. The patient is operated upon in the supine position with the neck in a minimally extended position. This extension is maintained either only by skull traction ,or by the addition of a soft roll at the nape of the neck. Hyper extension is to be avoided. The head is rested on a head ring of appropriate size and is kept straight and preferably not turned to either side. Some surgeons however prefer turning the head to the side opposite the side of approach to make the anterior border of the sternomastoid muscle prominent. Skull traction is applied before intubation and maintained through the surgical procedure.