Answer to the “Letter to the editor” of Dr. Wolfgang Börm

Dear Dr Borm, We would like to thank you for your comments that show you took great interest in our paper. When the first report on stand-alone lumbar cages came out, this method was obviously hailed as the answer to treatment of axial low back pain. However, further follow-ups and critical analyses showed that stand-alone lumbar interbody cages subsided, did not restore anatomic lumbar lordosis and had pseudarthroses. Some authors recommended that lumbar cages should not be inserted into the lumbar spine in a stand-alone fashion. However improved cage technology and further refinements to the surgical techniques have ensured that lumbar cages are here to stay. The biomechanics in the cervical spine are somewhat different and one would expect to see fusion more easily in the cervical spine than in the lumbar spine as simple cervical discectomy leads to fusion in a significant number of cases. Artificial discs that are supposed to maintain motion in the cervical spine have also been associated with spontaneous fusion in a number of cases. You criticise our paper because we do not give any clear conclusions as to the use of stand-alone cages in the cervical spine. As we have already written, our series was small and specifically studied subsidence of the cervical cages inserted in a stand-alone fashion even though the outcome of our patients was favourable in most cases. At the time of writing the paper, we did not have any of the most recently available literature on cage subsidence. A review of the most recent literature on the subject (including the two papers you quote) shows that it is recognized that cage subsidence may occur after the insertion of stand-alone cervical cages, and we are happy to have been the first ones to have brought this up in the form of a warning (20% rate of cage subsidence in the Payer series, 35% rate in the Thome series, 50% rate in the Kanayama series, 62% rate in our series) [1, 2, 3]. Payer is currently changing the size of its cage and its design because of the high rate of subsidence. The available literature on anterior cervical discectomy with or without fusion mostly reports excellent clinical results with resolution of the symptoms. The purpose of adding an instrumentation or interbody cage after anterior discectomy is threefold: to achieve a better fusion rate, restore anatomic segmental lordosis and restore intervertebral disc height. The findings of all the available literature and our series showed that no segmental motion was present on the flexion-extension X-rays after the use of stand-alone cervical cages. So we can therefore assume that stand-alone cervical cages fuse in the large majority of cases, as we were able to assume in our series. As far as segmental lordosis is concerned, there is obviously not enough information in the literature, and it is possible to debate the value of cylindrical or rectangular cages that do not restore anatomic lordosis. Lastly, subsidence of stand-alone cervical cages is now a well recognized problem in the literature. This rate of subsidence does not seem to affect the initial clinical result although one of the targets of the cage (restoration of disc space height) is not achieved in a large number of cases. However, all these clinical results are short-term follow-up results, and anterior discectomy alone or non-instrumented anterior fusion with dowel graft led to segmental kyphosis and loss of height in a significant number of cases. Such adverse effects were sometimes only observed after one or two decades after the initial surgery. Therefore we do not condemn cage technology in the cervical spine, but maintain our initial warning that many available stand-alone cervical cages may be associated with subsidence, as is now well documented in the literature. Long-term follow-up of such subsidence should determine whether this has a clinical impact on the patient’s symptoms and adjacent segments of the cervical spine. Lastly, I would like to urge you not to feel guilty about your previous use of anterior cervical cages. I suggest you document your cases and look at the clinical outcome of your patients, as well as the radiographic anatomic parameters of segmental lordosis, intervertebral disc space height and cage subsidence.