In this period, thirty-seven patients were surgically treated for a herniated disk causing radiculopathy (N=32) and/or myelopathy (N=5). Twenty patients were males. The mean age at the index surgery was 49.4 ± 8.5 (32.0-70.0) years. Thirty-four patients could be contacted. One man had been deceased seven years after surgery (he did not receive other surgical treatments for the neck except for the laminectomy after ACDF, see below), one male patient had moved towards another country, and one male patient refused participation. The mean period of follow-up was 9.6 ± 1.2 (7.6 – 12.1) years.

The levels of the index surgery are represented in Table 2. Fifty carbon fibre cages (Depuy Spine, Amersfoort, The Netherlands) were used in thirty-seven patients. All cages had a standard size. The height of the cages is shown in Table 3.

Figure 1

Table 2: levels incorporated at the index surgery (N= 37)

Figure 2

Table 3: cages used in 37 patients and 50 levels

At the last follow up, twenty-five patients (74%) had a good to excellent result, eight (24%) a moderate result, and one (2%) a fair result. Thirteen patients did not have any complaints anymore, twelve only minimal complaints, and eight had on a daily base complaints but did not need any medical intervention. One patient still had severe complaints but less than preoperatively. These complaints were not new but maintained after the surgery. He could manage the situation very adequately, and sometimes he consulted an anesthesiologist. None of the patients mentioned worsening of the symptoms compared to the preoperative situation. This summary of the results included also the patients that were operated twice.

Six patients underwent repeated surgery for neck complaints. The median time between index surgery and second surgery was 1.8 ± 1.7 (0.2 – 4.9) years. The reasons for the surgeries were: degeneration of the adjacent level (n=3), persistent myelopathic symptoms and signs (n=1), recurrence of radicular pain due to osteophytic compression at the same levels as were treated at the index surgery (n=1), luxation of the cage with difficulty of swallowing (n=1).

In the patient with a luxated cage, the cage was re-implanted and a fixation with a plate (Atlantis, Medtronic, Heerlen, The Netherlands) was performed. Prior to the index surgery, this patient had had a cervical laminectomy because of a intramedullary located ependymoma that had been removed successfully. One patient noted recurrence of the same complaints as preoperatively due to osteophytic spurs that compressed the nerve roots. A dorsal cervical foraminotomy was successfully performed. In another patient the result of anterior decompression for myelopathic changes was not satisfying. He did not mention any relief of the myelopathic complaints. New radiological examination still disclosed a stenotic spinal canal and he underwent a laminectomy. Afterwards, he noticed a slow but gradual improvement.

ADD was treated by cervical anterior with discectomy and implantation of a standalone cage. Revision of the original radiological exams learned that in two (66.6%) patients the adjacent discs had already signs of degeneration.

One patient suffered from hoarseness related to the index surgery. None of the patients complained of dyspaghia (expressed as discomfort when swallowing).

The sum of the years follow – up for this study or the moment of re- surgery for ADD for the 34 patients was 312.0. Only in one case the adjacent level was degenerated, whereas it looked radiographically completely normal before the index surgery. Therefore, the annual rate for ADD is calculated as (1/312) x 100% is 0.32 %.

Although ADD was related to a surgical procedure, knowledge of the normal ageing is warranted for interpreting the prevalence of ADD.

The roentgenographic ten years follow – up of originally asymptomatic persons has been described(4). Thirty of the 157 persons did not have initially degeneration but developed it at the follow up, seventy had a progression of an already present degenerated disc, and fifty-seven persons did not reveal any signs of degeneration at follow up. The last category were younger (44 years) than the other two categories ( 48 and 52 years, respectively). Fifteen percent developed pain. Only one patient was operated because of cervical radicular pain.

Recently, the MRI findings in 223 initially asymptomatic volunteers with a follow up of 10 years are represented(5). The age at the initial MRI was 39.0 ± 15.0 years ( range 11-71 years). During this period 9.9% of the participants complained of neck pain, 30.0% of stiff shoulders and 4.0 % of numbness in the upper extremities. 34.1% of the subjects had one or more clinical symptoms. None of them underwent cervical spine surgery. In 84.8% of the participants progression of degeneration at one or more levels was seen.

The same authorgroup compared in another study(6) patients who underwent an ACDF, the adjacent segments to the corresponding levels in the above mentioned group of asymptomatic volunteers. Sixty - four patients underwent ACDF, and were compared with 201 asymptomatic volunteers from the previous discussed cohort. The authors concluded that patients with an ACDF had a significantly higher incidence of progression of the degeneration at the adjacent segment. However, the baseline characteristics were not comparable. The ACDF group was significantly older. From the previous study it was known that progression of degeneration differed amongst age groups. Other possible confounding factors were not investigated. Therefore, comparison of these two groups did not validate firm conclusions.

Since progression of existing degenerative disc disease is related to the natural phenomenon of ageing, ADD should be restricted to newly developed degeneration of the adjacent disc. For the patient is only symptomatic ADD relevant. Therefore, our attention will focus on ADD for which the patients sought medical attention.

The literature was reviewed performing a Pubmed search till January 1 ^st, 2011 was performed using the following search string: ((adjacent segment degeneration) OR (adjacent disc degeneration) OR (adjacent disc disease)) AND cervical. In a second search the following string was added: AND ((arthroplasty spine) OR ((disc or disk) prosthesis)). The resulting number of articles found by the literature search is shown in Figure 1. It revealed a clear correlation between the terms adjacent disc degeneration and cervical arthroplasty. Pearson´s correlation coefficient was 0.993 and highly statistically significant (p< 0.0001).

Figure 3

Figure 1: Number of articles retrieved by a Pubmed search till January 1 , 2011. ADD represented the number of articles dealing with cervical adjacent disc degeneration. ADD + arthroplasty disclosed the number of articles dealing with cervical adjacent disc disease in combination with arthroplasty.

A very often cited article is written by Hilibrand et al(7). An annual rate of ADD after a CADF of 2.9% was calculated. However, the authors mentioned a large number of patients that were lost to follow up. Correcting for this and in a best case scenario (nobody of the missing persons suffered from a symptomatic ADD) the annual rate would reduce to 1.3%. It was also stated that 69% of the patients developing ADD had at the first surgery already signs of a degenerated disc. It merely was a progression of an existing problem. He recommended treating those levels also at the first surgery. Therefore, a new calculation estimated an annual rate of ADD after CADF of 0.4%.

Gore et al. reported a series of 48 patients with an average follow up of 21 years(8). Radiologically, all patients developed postoperatively ADD. In sixteen patients symptoms recurred differing from solely neckpain to radicular pain. Eight patients did not have any surgery for the recurrence. Only one of them had radicular pain. The other eight patients underwent additional surgery. Of them seven suffered from radicular pain. The mean time for recurrence of the complaints was 3.5 years for those receiving additional surgery, and 10.8 years for the patients not been operated again. Since it is debatable if neckpain can be contributed due to ADD, only radicular pain is considered relevant to symptomatic ADD. An annual rate of 1.5% after 11 years and 0.8% after 21 years can be calculated for ADD warranting additional surgery. The annual rate is calculated based on mean and not actual follow up. However, the surgical series included patients treated between 1968 and 1975. Surgical techniques and implants have greatly been evolved since then.

In another study by Gore et al.(9) the results of a larger sample are reported. 133 Patients were followed for a mean of 5 ± 3.3 years. Eleven underwent additional surgery because of clinical symptomatic ADD. The time between the index surgery and the resurgery was not provided. The patients were retrieved from 1961 till 1981.

Another frequently cited article is written by Goffin et al(10). Degeneration of adjacent segments was found in 92% of the cases with a mean follow up of more than five years (8.4 ± 2.6 years) after the index surgery. However, of the living patients only 55.1% contributed to the study. The number of ADD is probably less than the reported 92%. Of the studied patients, 36.7% was operated because of a traumatic lesion of the cervical spine. A traumatic event does affect the whole cervical spine including the segments adjacent to the injured segment. The influence of the trauma is not similar to degeneration, and those patients should be excluded. Plate fixation was used when a corpectomy was performed or when fusion should be achieved at more than two disc levels. Therefore, it can be concluded that different techniques are used for different diseases. Certainly this will influence the number of ADD. Finally, the authors did not mention how many patients did not suffer from any degeneration at the index surgery. If ADD is defined as developing new signs of degeneration the number is presumably much less that 92%. It is remarkable that only 11 (6.1%) patients of 180 patients were operated again because of ADD, or 11 (3.4%) of 327 still living patients. In other words, with a mean follow of 8.4 years 0.73% of the patients will need a resurgery because of symptomatic ADD after a CADF or 0.4% in a best case scenario. It should be explicitly stated that the population is different from those suffering from simple degenerative, one or two level disc disease. Would this have been the case, the annual rate would probably have been lower.

Arthroplasty was considered to prevent ADD. More and more studies are published reporting the results of arthroplasty.

Recently, the long-term follow up of a disc prosthesis was reported(11). Each patient was followed at intervals of two years. Eighty-nine patients completed six years follow-up. In this time four patients were operated upon because of clinically symptomatic ADD. So, 4 patients with ADD were seen in a total of 534 years follow-up, resulting in an annual rate of 0.75 %. If these patients are followed for 8 years, and none of the other patients will develop ADD the annual rate will fall to 0.56 %, and to 0.44 % at ten years follow up. These results closely resembled the recalculated rates from the previous studies. It should be noted that the annual rate of ADD differed with the time of follow-up. To compare annual rates, the follow up should be the same.