Lumbar arthrodesis for degenerative conditions of the spine has come under exacting scrutiny over the past few years. Reviews of previously published studies have suggested only modest improvements in results despite increased use of technologies such as pedicle screws and interbody devices1,2, and these results have raised doubts about the benefits of fusion for patients affected by the wide spectrum of disorders categorized as "degenerative disc disease."
It has been difficult, at best, to evaluate the results of lumbar arthrodesis because of the lack of evidence-based studies. Despite the use of a variety of accepted, validated, and disease-specific instruments, the clinical relevance of statistically significant improvements in outcome and pain scores has been increasingly contested. Consider the following illustrative example. Study X found that Oswestry Disability Index scores were improved, on average, from fifty to forty. With a large enough sample size, this ten-point improvement can be statistically significant with a p value of <0.05. However, it is unknown if a ten-point improvement in the Owestry Index represents a clinically significant improvement for the patient.
The recognition of this disconnect between statistical and clinically meaningful differences has had a justifiably pervasive impact in the spinal literature. Thus, the concept of a minimum clinically important difference has been applied3. Notwithstanding its importance, a minimum clinically important difference refers to the proverbial "bare minimum" change that a patient would perceive as an improvement in pain, function, or other assessed parameter. This method of assessment does not take into consideration what a patient would consider to be an improvement sufficient to elicit satisfaction with the outcome of the operation.
These outcome deficiencies are precisely what Glassman et al. have attempted to answer in their study. Moreover, the investigators confined themselves to a group of patients who had undergone treatment for degenerative lumbar disorders, the diagnostic group most susceptible to interpretative challenges. Their efforts should be applauded.
Glassman et al. have translated a patient's report of "mostly satisfied" or "much better" into a numerical change in the Oswestry Disability Index, Short-Form-36 (SF-36) questionnaire, and visual analog scale pain scores. By doing so, one would hope that the reverse process might work equally as well—that is, that the substantial clinical benefit parameters could be applied post hoc to previously published and future outcome studies.
The current proposal of seemingly exact numerical representations of substantial clinical benefit should be cautiously interpreted, however. While Glassman et al. have suggested, on the basis of their data, that a net change of 2.5 in leg or back pain (according to Table IV in their paper) can distinguish between feeling "much better" or "about the same," the threshold between these two patient-reported conditions is likely much more complex. It is my impression that a reduction of pain by only 2.5 is ostensibly a small number. In counseling a patient prior to surgery, it would seem an overestimation to think that most patients would consider such a change to be clinically substantial. Beyond the magnitude of change, one might also question the equivalence of a change from disparate starting levels of pain. For example, could (or should) one conclude that a 2.5-point change from 10 to 7.5 is equivalent to a change from 6 to 3.5?
Another observation is that the range of pain, Oswestry Disability Index, and SF-36 scores in the compared groups had considerable overlap. In their Table III, Glassman et al. documented that the average SF-36 scores and standard deviation for the "much better" and "about the same" groups were 13.8 ± 9.3 and 2.4 ± 6.1, respectively. Thus the calculated ranges, according to the standard deviations, would be 4.5 to 23.1 for the "much better" group and −3.7 to 8.5 for the "about the same" group. The upper and lower limits of these groups have substantial overlap. As the authors state in the Materials and Methods section, it is perhaps the value for the area under the curve (in this case 0.846) that is more meaningful, in that it implies that there was, on the average, an 84.6% chance that patients with an SF-36 change of at least 6.2 were truly "much better" than the "about the same" group.
As a final observation, the current study suggests that each of the three outcome thresholds (SF-36, Oswestry Disability Index, and pain score) is an independent criterion for substantial clinical benefit. However, the three are likely to be interdependent. Future work might be directed at formulating a combination of criteria that would more accurately represent the full spectrum of what a patient interprets as a real or substantial clinical improvement as a result of surgery.
In summary, Glassman et al. have produced a worthwhile contribution to the literature regarding an issue that is becoming increasingly crucial in the assessment of use of lumbar fusion for degenerative conditions. As they are the first group to suggest a criterion threshold for substantial clinical benefit, I would hope that their group continues their work, perhaps defining influential covariables, such as individual degenerative subgroups.
*The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated.
1. Bono CM, Lee CK. Critical analysis of trends in fusion for degenerative disc disease over the past 20 years: influence of technique on fusion rate and clinical outcome. Spine. 2004;29:455-63; discussion Z5.
2. Gibson JN, Waddell G. Surgery for degenerative lumbar spondylosis: updated Cochrane Review. Spine. 2005;30:2312-20.
3. Copay AG, Glassman SD, Subach BR, Berven S, Schuler TC, Carreon LY. The minimum clinically important difference in lumbar spine surgery patients: a choice of methods using the Oswestry Disability Index, Medical Outcomes Study questionnaire Short Form 36, and Pain Scales. Spine J. 2008 Jan 15. [Epub ahead of print].