Cell Senescence and the Human Intervertebral Disc
H.E. Gruber, PhD; E.N. Hanley, Jr., MD; H.J. Norton, PhD

Cell senescence has been recognized as a potential cause of disc degeneration, but little basic research has addressed this topic.  Senescent cells are viable and metabolically active but they cannot divide. Senescent cells have altered patterns of gene expression, altered responsiveness to external stimuli, and altered interaction with the extracellular microenvironment.  Senescence in the disc is important since  it may reduce the disc’s ability to generate new cells to replace existing ones lost to necrosis or apoptosis. It is also possible that the products of gene expression of senescent cells may effect disc physiology.  The following paragraphs summarize our findings to date on in vivo and in vitro experiments.

1)  Increased cell senescence is present in vivo in the human disc in association with decreased cell proliferation.  The objective of this study was to determine the relationship in vivo between disc degeneration, the proportion of senescent cells, and the proportion of proliferating cells in the human annulus.  Studies were approved by our human subjects Institutional Review Board. Annulus tissue from 26 discs from 24 subjects was studied.  22 were surgical patients and 2 were control donors. There were 24 lumbar specimens and 2 cervical specimens. Distribution according to Thompson grades was:  1 grade I disc, 4 grade II discs, 9 grade III discs, and 12 grade IV discs.  Mean subject age was 42.3 years + 14.3 (24) (mean + SD(n)).  Immunohistochemical localizations were performed to detect the Ki-67 antigen which marks proliferating cells (Fig. 1A), and senescence-associated beta-galactosidase which identifies senescent cells at pH 6.0 (Fig. 1B).  (Arrows mark cells negative for the immunocytochemical localizations.)  A significant negative correlation was present between the % senescent cells vs the % proliferating cells (Fig. 1C), r = -0.013, p = 0.013.  In addition, more degenerated grades III and IV discs contained significantly greater percentages of senescent annulus cells than did the healthier grades I and II discs (44.4% + 20.0 (21) vs 18.8% + 11.0 (5), respectively; p = 0.011).  There was no significant difference for the % proliferating cells for these two groups (4.7% + 1.6 (21) for grades III and IV vs 5.3% + 1.9 (5) for grades I and II).   No correlation was present between age and the % senescent cells or age and the % proliferating cells.

2) Exposure of human annulus cells to hydrogen peroxide (H₂O₂) in vitro  is a reliable, reproducible model of stress-induced senescence:  Oxidative stress has been used previously to induce senescence in other cell types, but to date there have been no studies to see if stress-induced premature senescence could be induced in human  annulus cells by H₂O₂ exposure.  As shown in Figure 2, exposure to 50 µM. H₂O₂ for 2 hours results in a significantly elevated incidence of senescence 4 days post-treatment, p <0.0001.  An example from a culture of H₂O₂-treated annulus cells with senescence-associated beta-galactosidase localization in senescent cells is shown in Figure 3A (Fig. 3B shows the relevant negative control).

In summary, results identified to date:  1) Help us better understand the quantitative presence of senescent cells and proliferating cells within the human annulus.  A significant negative correlation was present between the proportion of senescent and proliferating cells, such that increased cell senescence is present in association with decreased cell proliferation in vivo; 2) Verify that stress-induced premature senescence  induced in human annulus cells by H₂O₂ exposure is a reliable and reproducible experimental model; studies are now underway during the last portion of this grant to test the ability of selected growth factors to rescue disc cells from senescence in vitro.

We thank the North American Spine Society for funding our research on cell senescence in the human intervertebral disc.  This project is now complete.  Major findings are as follows:

1)  Rescue of human disc cells from cell senescence:  (In press, Growth Factors, 2008)

The aging human intervertebral disc contains a sizeable population of senescent cells.  Since senescent cells cannot divide, senescence reduces the disc’s ability to generate new cells to replace existing ones lost to necrosis or apoptosis.  The objectives of this portion of our work were  1) To develop a reliable in vitro model for stress-induced premature senescence in human annulus cells, and 2) To investigate the potential for IGF-1 to prevent or ameliorate senescence in vitro.  The developed experimental model employs a 2 hour exposure to 50 µM H₂O₂; immunocytochemical localization of senescence associated-beta-galactosidase (SA-ß-gal) at pH 6.0 was used as the marker for senescent cells, and the percentage of senescent cells quantified after 3 days of culture. Nine sets of annulus cells were obtained from 8 human surgical disc specimens; cells were tested with 0, 50, 100 or 500 ng/ml IGF-1.   Although 50 ng/ml or 100 ng/ml IGF-1 did not significantly alter the percentage of senescent cells, a significant reduction was present following exposure to 500 ng/ml IGF-1 (control, 56.3% + 8.5 (9) (mean + sem, (n) vs treated, 39.6% + 6.6 (9), p = 0.0009).  These novel findings point to the value of continued research towards development of future biologic therapies designed to reduce cell senescence in degenerating human discs.

2)  Increased cell senescence is associated with decreased cell proliferation in vivo:  In press, The Spine Journal, 2008.

 A second major study was carried out to determine the percentage of proliferating and senescent cells in the aging and degenerating human disc.  Immunohistochemistry was utilized to detect senescent cells using an anti-senescence-associated beta-galactosidase antibody, and an anti-proliferation antibody (Ki-67).  An average of 410 cells/specimen were counted to determine the percent senescence, and an average of 229 cells were counted to determine the percent proliferation.  Cell proliferation was low in both surgical and control normal donor annulus tissue (4.09% + 1.77 (26), mean + SD (n)).  There was no significant difference in the percentage of proliferating cells for more degenerate discs vs healthier discs (4.7% + 1.6 (21) for grades III and IV vs 5.3% + 1.9 (5) for grades I and II).  More degenerated grades III and IV discs contained significantly greater percentages of senescent annulus cells than did the healthier grades I and II discs (44.4% + 20.0 (21) vs 18.8% + 11.0 (5), respectively; p = 0.011). A significant negative correlation was present between the percentage of senescent cells vs the percentage of proliferating cells, r = -0.013, p = 0.013.     No correlation was present between age and the percentage of senescent cells or age and the percentage of  proliferating cells.  Since senescent cells cannot divide, senescence may reduce the disc’s ability to generate new cells to replace cells lost to necrosis or apoptosis.  Senescent cells also accumulate in the disc over time, such that their metabolic patterns may contribute to the pathologic changes seen in degenerating discs. Novel data presented here show a significant negative correlation between the percentage of senescent cells and the percentage of proliferating cells during disc degeneration.  Molecular work is underway in our lab to help us determine whether senescent cells in the disc secrete factors that can result in decreased proliferation in neighboring cells. 

Local Delivery of Small Inhibitory RNA (siRNA) and Radiation Therapy to Treat Metastatic Spine Tumors
D.M. Sciubba, MD; Z.L. Gokaslan, MD, FACS; T.L. DeWeese, MD

Overall Hypothesis
Locally delivered plasmid based small inhibitory RNA (siRNA) will increase the radiosensitivity of breast cancer in a metastatic spine tumor model. To test this hypothesis we will use an siRNA to suppress DNA-PK, a key protein in DNA repair. We hypothesize successful DNA-PK siRNA transfection, in combination with standard radiotherapy, will decrease tumor growth and prolong the onset of paraparesis in rats challenged with a lethal dose of intravertebral breast carcinoma cells.

Aim 1. Determine the dose-response of siRNA DNA-PK in CRL-1743 breast cancer cells.

1. Successful transfection of CRL-1666 and 1743 cells was achieved using GFP as a control transgene (Figure 1).
   
   
2. Following irradiation of non-transfected cells, cells transfected with a scrambled plasmid, and cells transfected with the siRNA plasmid, we have noted a decrease in the amount of protein expression of DNA-PK in the treated cells (Figure 2).
   
   
3. In addition, following irradiation of non-transfected cells, cells transfected with a scrambled plasmid, and cells transfected with the siRNA plasmid, we have noted an increased radiation-associated killing in the treated cells

Aim 2. Establish the time course of breast cancer cell killing, progression to paraparesis, and overall survival in tumor-bearing animals following radiation to the spine (15 Gy) with and without concomitant DNA-PK siRNA.

1. We have implanted breast cancer cells into the vertebral body of rats, consistent with our previously established model, and followed their BBB scores (neurological assessment) and survival in untreated animals and in animals following standard radiation.
   
   
2. Based on preliminary work of our colleages in the lab, adenovirus has been shown to successfully transfect plasmids similar in size to ours to rat brains in vivo following brain tumor implantation. However, this has not worked well with our plasmid. As a result, we have recently used liposomes. We are now currently using this technique to continue transfecting rats. 
   
   
3. Early successful in vivo transfection has been demonstrated, and we are currently assessing the neurological outcomes and survival benefits of rats with breast cancer lesions of the vertebral body treated with transfected siRNA to DNA-PK compared to controls. We will continue such work in our laboratory. Notification of all accepted publications and presentations will be sent to NASS regarding hopeful results.

Findings to Date

1. Small-inhibitory RNA (siRNA)-mediated silencing of DNA-PK leads to decreased protein levels and increased cell kill in CRL-1666 and 1743 breast cancer cells, leading to a theoretical potential technique for radiation-sensitization of metastatic breast cancer.
   
   
2. Current studies suggest that successful in vivo transfection via liposomes of plasmid-encoded siRNA to DNA-PK into rat spines is possible.
   
   
3. We are currently assessing the neurological outcomes and survival benefits of rats with breast cancer lesions of the vertebral body treated with transfected DNA-PK siRNA compared to controls.

The Regulation of Gene Expression to Compressive Stress in the Nucleus Pulposus
G. Sowa, MD, PhD; J. Kang, MD; P. Smolinski, PhD

Status of Research Progress: The goal of this project is elucidation of the signaling pathways initiated by compressive force and how the thresholds for these differ in healthy and degenerative discs to provide increased ability to predict what types of movements should be preserved and promoted for optimal disc health.  As summarized in the previous progress report, we have designed, fabricated, and validated a compression system capable of imparting 0-3000 psi of static compression onto cells or tissues. Since the submission of the progress report in 2007, we have expanded our measurements of gene expression changes to include additional genes as well as additional regimens as outlined in the findings below. In addition, we have performed experiments to determine the conditions under which native tissue may be tested in the compression chamber. We have confirmed in our own laboratory the use of 35% polyethylene glycol in the exterior chamber prevents the swelling/osmotic gradient previously preventing testing of native tissue, facilitating future testing of native tissue and comparisons to the cell based studies presented here. We have also begun development of a novel dynamic control system to allow measurement of compression of various frequencies in addition to the reported static compression.

Findings and Conclusions to Date:
We have completed testing of the effects of various magnitudes and durations of compression on gene expression in nucleus pulposus cells as outlined in the attached abstract. Increasing magnitudes showed more beneficial gene expression changes, while increasing duration resulted in gene expression changes consistent with traumatic effects, as summarized in the chart below.

In addition, we have performed analysis of the combined effects of inflammatory and mechanical stimulation through examining the gene expression response of nucleus pulposus cells exposed to both IL-1beta and compression after both 4 and 24 hours (n=2).  The observed trends suggest that after 4 hours, low levels of compression decrease the expression of iNOS but not COX-2. However, after 24h, low levels of compression do not demonstrate this effect and actually potentiate the increase in iNOS and COX-2, while higher levels of compression are now inhibitory of the inflammatory response (see graphs below).  Further replicates will be needed to confirm these observed trends.

We have also examined effects of compression on matrix turnover through measurement of CS846, a breakdown epitope released during aggrecan biosynthesis. We have demonstrated overall trend toward decrease in the exposure of this epitope after 4 hours of compression at 600psi, and increase after 24 hours at 600 psi. Interestingly, this is consistent with the trends in aggrecan gene expression under these conditions. Future studies will examine the expression of this aggrecan epitope under conditions of various magnitudes of compression.

In addition, we have completed  fabrication and preliminary testing of the dynamic feedback control system for actively controlling cyclic loading of the pre-existing compression chamber.  The dynamic feedback control system is driven by a linear actuator (Ultramotion, D-B.125-HT23E10DIFF-4-/4) which has a maximum load capacity of 260 lbs and maximum velocity of 5 inches/second.  The linear actuator is coupled with a hydraulic cylinder (McMaster-Carr, 6297K11) and a digital pressure transducer (Omega, PX309-1KG5V).  All components are connected to a PC and controlled via a custom written MATLAB programmed with a Proportional-Integral-Derivative (PID) controller.  The PID controller is still being optimized; however it is hypothesized, based on theoretical calculations, that the final system will be capable of operating stably at approximately 5 Hertz enabling simulation of physiologic levels of pressure magnitude and frequency.  Therefore, future experiments will be focused on examining how various frequencies of compression contribute to the observed gene expression changes.  

The Minimal Clinically Important Difference (MCID) for Spinal Disorders: Finding the Threshold of Clinically Significant Change
V. Deviren, MD; S. Berven, MD
This grant was funded for one year only and the following are the results from that year.

STATUS OF RESEARCH PROGRESS: Currently, we have collected data from 115 consecutive lumbar patients and 43 consecutive cervical patients who had undergone operative treatment for degenerative spinal disorders.  We are continuing enrollment at this time. 

BACKGROUND CONTEXT: Patient-reported outcomes instruments, including the Medical Outcomes Short Form 36 (SF-36) and the Oswestry Disability Index (ODI), are commonly used to evaluate the effectiveness of spinal surgery. As we begin to utilize patient-based health status outcomes as a measure of utility for a given intervention, it becomes important to differentiate between statistically and clinically significant changes in health status.  The minimum clinically important difference (MCID) represents the smallest improvement considered worthwhile by a patient.  The concept of the MCID is offered as a new standard for determining the effectiveness of a given treatment and describing patient satisfaction in reference to that treatment.

PURPOSE:
The primary aim of this study is to establish the threshold at which the MCID occurs for patients with variety of degenerative disorders on the SF-36 and ODI.  The maximum clinically important difference (MaCID), representing a major improvement in health status as recognized by the patient, is also be examined.  

METHODS: This is a prospective, observational study.  The primary means of determining the MCID for patients are divided into anchor-based and distribution-based methods.  Outcomes data SF-36, Oswestry Disability Index, General Self-Assessment (14-item questionnaire), and Visual Analog Scale (VAS) were analyzed using both methods.
Changes in outcomes scores over time (baseline, 6 weeks, 3 months, 6 months, 1 year, 2 years) were compared.  The MCID for function, as measured by the ODI, and separate MCIDs for general well being, pain, mood, and relationships, as measured by the SF-36 total score, the Bodily Pain Domain of the SF-36, the Mental Health Component Score of the SF-36, and the Social Function Domain of the SF-36, respectively, were established for patients with degenerative spinal disorders.  Two different techniques (anchor-based and distribution-based) were used to establish the MCID and these results were compared. 

In this study we used 15-item Likert scale questionnaire (See Figure 1) to evaluate treatment effect as global assessments of change and to provide an anchor for domains of general health, pain, mood (mental health), relationships (social function), and physical function.  We developed a visual analog scale (VAS) (See Figure 2) for each domain in order to obtain a more subjective anchor to differentiate the MCID.

  
 Figure 1                                                                         Figure 2 

Using the anchor-based technique for determining the MCID, we compared observed changes in the SF-36 and ODI scores from pre-treatment to post-treatment with patient self report change assessments.  For the Likert scale we defined a positive minimal clinically significant difference as a change that is recognized as “a little better”,  “somewhat better”, and “moderately better” by the respondent and a negative minimal clinically significant difference as a change that is recognized as “a little worse”, “somewhat worse”, and “moderately worse” by the respondent.

 To validate the use of retrospective self-assessment questionnaires (GAQ, VAS) in assessing change in health status, we compared the mean changes in health status for those self-assessing improvement, no change, and worsening of health.  Rank correlation analysis (Spearman’s rho) and linear regression was performed separately for each MCID domain (well-being, pain, function, mood, and relationships) on the GAQ and VAS in order to validate the use of each domain. 

Using two distribution based methods (SEM, 95 % confidence interval, and standard deviation), we measured the MCID.  Using the 95% confidence interval method, the MCID for improvement and deterioration was defined as a range, with the lower confidence interval of the group with minimum change being the lowest estimate, and the highest confidence interval of the group reporting no change being the highest estimate.

We utilized the standard error of measurement (SEM) as a second distribution-based method to assess the MCID for our patient groups.  The SEM is calculated using the formula:       
SEM= σx√ 1-ru

where σx is the standard deviation of a HRQoL measure and  ru is the reliability coefficient.  The advantage of using the SEM is that it, unlike effect size or the ratio of difference to variance, is not reliant on the sample, i.e. it is consistent across the entire range of a measurement. [16] Wyrwich et al. has found that 1 SEM criterion corresponds to external anchors for MCIDs for various HRQoL questionnaires for a variety of different diseases. [16]

PILOT RESULTS: Outcomes data was prospectively collected from 115 consecutive lumbar patients and 43 consecutive cervical patients who had undergone operative treatment for degenerative spinal disorders.  See Table 1a and 1b. The average age of the lumbar cohort was 55 (range 17-82), and was comprised of 61 females and 54 males.  The average age of the cervical cohort was 52 (range 32-87), and was comprised of 29 females and 14 males.  Datapoints were collected at baseline, 6 weeks (n, cervical=14; n, lumbar=40) 3 months post-op (n, cervical=15; n, lumbar=38), 6 months post-op (n, cervical=14; n, lumbar=36), 1 year post-op (n, cervical=6; n, lumbar=31), 2 year post-op (n, cervical=2; n, lumbar=3).

To assess the validity of our VAS questionnaire as a potential anchor, we ran Spearman two-tailed correlation analysis for the VAS questionnaire and the 15-item Likert scale for overall well-being, pain, function, mood (mental health), relationships (social function).  We found a significant correlation between the two scales in each of the domains (Likert Well-Being/VAS Well-Being r=0.60, Likert Pain/VAS Pain r=0.73, Likert Function/VAS Function r=0.77, Likert Mood/VAS Mood r=0.74, Likert Relationships/VAS Relationships r=0.58). 

In the lumbar cohort, at 114 datapoints patients reported experiencing an overall improvement from their pre-operative health status, while 10 patients reported experiencing no change and 20 reported overall worsening (Table 3a and 3b). In the cervical cohort, at 32 datapoints patients reported experiencing an overall improvement from their pre-operative health status, while 9 patients reported experiencing no change and 10 reported overall worsening (Table 3c and 3d).  We ran Spearman two-tailed correlation analysis and reported only the moderate or greater r values for the correlations between the SF-36 domain scores/ODI/NDI scores and the GAQ and VAS self-assessment ratings. listed in Table 2.

Using the patient self-report 15-item questionnaire as an anchor, the MCID for general well-being, pain, function, mood, and relationships were measured. Patients reporting a minimum improvement (≤4 points) were included in the analysis.  Each domain of the MCID was assessed according to the score that was most strongly correlated with (if the correlation was statistically significant).  Therefore, the determination of the MCID for general well-being, as measured on a 15 point scale, was determined using the change in SF-36 total score.  For pain, the subjects’ self-assessment was determined using the Bodily Pain domain of the SF-36.  For  function, the MCID was determined using the change in ODI total score for the lumbar group and the PhyFcn SF-36 domain for the cervical group.  For mood, the Mental Health component of the SF-36 was used to determine the MCID.  For relationships, the Social Function Domain of the SF-36 was used to determine the MCID for relationships, although there was not even a moderate correlation. Only patients reporting improvement in a specific domain were included in the analysis of that domain.  If the direction of change in outcomes scores conflicted with the direction of change of patient self-report, subjects were excluded from the analysis.  We compared patients reporting a minimum positive change (≤4 points) with patients reporting a maximum positive change (≥5 points).   The values for the MCID and the MaCID are reported in Table 4a and 4b.

DISCUSSION: Our findings suggest that the value of outcomes change scores that reflect a minimal clinically important difference from the perspective of the patient are, indeed, small.  Our results demonstrate, importantly, that the MCID should be determined in regard to specific domains, as the majority of patients reported different scores in each domain (well-being, pain, function, mood, relationships) and the MCIDs differed according the domain examined.  While the relative importance of each specific domain can be compared to the overall well-being change rating, it would be interesting to examine these relationships in further detail.  Although our data collection is not yet complete and we have not yet reached sample size in each domain, we found it interesting to discuss some of our preliminary findings in this pilot report. 

Our sub group analysis (lumbar/cervical) show that the use of the VAS as an anchor is feasible and may be best used when accounting for global change in health status not related to a specific disease state.   The VAS scores were more strongly correlated across the board to changes in SF-36 outcomes scores than the 15-item assessment (which allows more freedom than the typically used 6 or 7 item questionnaires).  The use of the VAS as anchor eliminates one important criticism of the anchor-based method; that the meaning of change differs according to the arbitrary divisions of the anchor’s scale.  Although, conceptually, the MCID is the difference between a “no change” category and a “minimum change” category on a scale, actually evaluating the MCID in this manner is problematic and self-limiting.  The use of a categorical anchor may not represent the patient’s actual change in health status, therefore use of a continuous anchor, such as the VAS, would be a better assessment tool.  Because the VAS is continuous, unlike a Likert scale, and is not subject to arbitrary divisions, the meaning of the MCID determined using a VAS as an anchor will not differ according to the level or number of divisions of the anchor scale.  One potential problem is that the level at which a minimum change vs. no change is delineated has not been established.  However, to be able to validate the use of the VAS, we still need further enrollment to reach our estimated sample size. 

While this preliminary data is able to serve as a starting point for determining the MCID for degenerative spinal disorders, much work remains to be done in order for a truly comprehensive and clinically useful MCID value to be determined.  Different methods of determining the MCID (anchor-based vs. distributional) yield vastly different results and it remains open to discussion which method is more accurate.

Several limitations of the data relate to the number of patients on which data was able to be collected.  We were able to collect data on 115 consecutive patients with degenerative lumbar conditions (See Table 1).  While we were able analyze the pilot data by combining all diagnoses and timepoints and report significant results, we feel that more data must be collected until we have reached the sample size predicted by our power calculation (n=38) for each diagnosis and timepoint.  Furthermore, to be able to form a complete picture of the MCID, we believe that the calculated sample size must be reached for each group (“no change”, “minimum change”, “maximum change”) in addition to diagnosis and timepoints.

We collected data on 43 consecutive patients with degenerative cervical conditions. Once patients were categorized as either “small improvement”, “no change” or “large improvement” the resulting groups sizes varied between 13-22 with disparate diagnoses and timepoints included in each group reporting minimal change.  More data should be collected to validate the results of the cervical group. 

Additionally, for both the cervical and lumbar branches of the study, too few patients reported deteriorations to be able to calculate MCID values for deterioration.  Although it would be beneficial to calculate an MCID for deterioration, it does not seem practical at this time.  As predicted, the number of patients reporting deteriorations is very small.  It would take an inordinate amount of time to amass a significant number of patients rating themselves as worse than baseline. 

The MCID is influenced by a wide variety of factors and it is important to be able to tease these factors apart. We were unable, due statistical power issues, to stratify patients based on diagnosis and surgery.  Additionally, we did not separately analyze the changes scores while controlling for the floor and ceiling effect exhibited in baseline outcomes scores.  We believe that the MCID is temporally dependent and will change over time.  Because of this, patients should be grouped based, not only on diagnosis, but also on time out from surgery.  In further work, it is important to determine the specific impact of baseline status, post-operative time point, and diagnosis on the MCID.  

A further consideration is the impact of the patient’s expectations at the outset of treatment and the impact pre-operative expectations on post-operative assessments of change.  While this is beyond the current scope of our project, it would be interesting to examine this in the future.

CONCLUSIONS:  An ideal means of determining the MCID for a given intervention is yet to be determined.  While we were able to establish MCIDs, these numbers may be subject to change in the future.  Our pilot MCID values are subject to change as more data is amassed and sample size is reached.  Additionally, because most of the follow-up reported on is short term (lumbar average= 4.49 months; cervical average= 5.38 months), subject reports of pain and function may still be influenced by the proximity of surgery and follow-up timepoint.  As subjects are further out from surgery (1 year, 2 year), we believe the outcomes data will not reflect pain and loss of function due to the surgery itself and will, therefore, be more accurate.  On the contrary, it may take longer to reach a significant sample size further out from surgery, as many patients may report significant improvement at later timepoints compared to earlier one.  

There are several limitations in the accurate determination of an MCID have been identified: 1) the multiplicity of MCID determinations (i.e. different methods yield different results)   2)  patients report significant changes early on in the post-op period and we predict will continue to report significant changes further out from surgery, making it take longer to enroll a big enough sample of patients reporting minimal change at each timepoint 3)  the dependence of the MCID on pre-treatment baseline status and patient expectations.  We believe it is possible to develop a useful method provided that the assumptions and methodology are initially declared. Our efforts toward the establishment of a MCID rely upon the establishment of specific external criteria based upon the symptoms of the patient and treatment intervention being evaluated.

Based on our preliminary results, an ideal means of determining the MCID for a given intervention is yet to be determined.  Even though the MCID should be applicable to any follow-up time point, subject reports of pain and function may be influenced by the proximity of surgery with short follow-up time point (≤ 6 months).  As subjects are further out from surgery we believe the outcomes data will be more evenly distributed. Several limitations are detected in the accurate determination of the MCID: the multiplicity of MCID determinations, conflicting patient self-report, individual perception, wide variation among patient’s scores, and the relationship between pre-treatment baseline and post-treatment change scores. 

 *Abstracts/permission forms not received at the time of publication
**Current and ongoing research