Outcome Assessment Of Occupational Carpal Tunnel Syndrome In A Cohort Of Truck Assembly Workers

Robin S. Baver, M.D., M.P.H. (A)
Al Franzblau, M.D.(B)
William B. Bunn, M.D., M.P.H. (A)

(A) Navistar International Transportation Corporation
6125 Urbana Road
Springfield, Ohio 45502

(B) The University of Michigan School of Public Health
Ann Arbor, Michigan 48109

ABSTRACT

The first objective of this study was to perform a detailed analysis of the outcome following treatment of occupational CTS in a well-defined cohort of truck assembly workers utilizing a specific case definition for cohort construction and detailed follow-up information. The computerized workers' compensation claims management data base and the factory workers' individual occupational medical records were used to generate the outcome assessment. The results were compared, qualitatively, to those previously reported in the literature. Finally, formal assessment of the return to work and restriction experience of those post-treatment for occupational CTS provided an estimate of on-going impairment and its impact upon the occupational functional level of the production workforce. The occupational CTS cohort consisted of 133 cases arising between 1/1/89 and 12/31/92. Greater than 97% of the cohort undergoing surgical release were able to resume their previous job following treatment. Generally, lost work time occurred only for post-operative recovery and most cases returned to work with either no or short-term activity restrictions. This experience is more positive than that reported for other occupational groups. Conservatively treated cases more frequently received activity restrictions and, in general, did not involve lost work time.

INTRODUCTION
Carpal tunnel syndrome (CTS) is the most commonly encountered compression neuropathy of the upper extremity, with annual cumulative incidence of 0.1% in the general population¹ and up to 5% in certain occupational groups². Occupational CTS is a major cause of lost work days and workers' compensation cost in the United States; carpal tunnel release is the most commonly performed operation on the hand, accounting for approximately 200,000 procedures per year in the United States and incurring direct medical expenses in excess of 1 billion dollars annually³. Scant data exist in the medical literature regarding outcome following surgical or conservative treatment of occupational CTS. The few studies assessing outcome have been limited by the lack of a specific case definition, relatively non-detailed follow-up information, and the use of general indices such as return to work (as a dichotomous variable) and duration of time loss as outcome measures. In one investigation of a meat packing plant, only 52% of those surgically treated had returned to their previous employment after one year⁴. A recent evaluation of occupational CTS outcome in Washington State revealed that 82% of surgical cases returned to either their same job or a different job⁵. Mean time loss post-operatively was 113 days, but 8% experienced time loss in excess of one year. Previous studies of occupational CTS outcome have reported only post-surgical experience and thus no data are available in the literature regarding outcome with conservative management of the condition. Additionally, no studies to date have assessed outcome with respect to restricted activity or on-going periodic disability following treatment.

METHODS
Description of Source for Study Population

Navistar International is a mid-size and heavy truck manufacturer. The Springfield Operations include two major manufacturing and assembly facilities, the Body and Assembly Plants. Cabs for all models of International trucks are fabricated at the Body Plant, from forming of basic metal cab componentry through stamping and welding operations to final assembly of the cabs. At the Springfield Assembly Plant, complete assembly of the International trucks, from basic components to completed product, is performed. The two facilities share a common hourly workforce numbering approximately 5000 which is represented by the United Auto Workers; this is an aged (mean age = 50 years), experienced (mean years company service = 27), predominantly male (96%) occupational cohort performing heavy manual processes which are repetitive, and often vibratory, in nature. Job placement is based upon seniority. An incentive piece work payment system is utilized in a portion of the Springfield Body Plant facility, further discouraging appropriate recovery time breaks and pacing of activities.

Construction of Occupational CTS Cohort and Outcome Evaluation

The cohort of Navistar Springfield Operations factory workers developing occupational CTS between 1989 and 1992 was constructed in the following manner:

The CTS case definition and inclusion/exclusion criteria for the study cohort were set.

The computerized workers' compensation data base was accessed to identify potential cases occurring between 1/1/89 and 12/31/92.

An individual medical record review for each potential case was performed to obtain clinical variables utilized in the evaluation of the case definition and inclusion/exclusion criteria.

Follow-up of each cohort member via individual medical record review, spanning from date of diagnosis to date of withdrawal from the cohort (secondary to death, termination, retirement, or long-term medical disability not related to CTS) or the study cut-off date of 3/31/95, was accomplished and included elucidation of lost and restricted work days on the claim, claim cost, return to work status, and the occurrence and timing of temporary or permanent work restrictions. Outcome measures were summarized for the entire cohort and then analyzed with stratification as to type of treatment received (conservative vs. surgical).

RESULTS
Cohort Characterization

The Navistar Springfield Operations computerized worker's compensation claims management system revealed that 140 claims for carpal tunnel syndrome were allowed from among the population of factory workers between 1/1/89 and 12/31/92. From the 140 potential cases, 7 were eliminated due to not meeting the case definition and/or the inclusion criteria; thus, the study population included 133 cases of occupational CTS. The 133 cases accumulated 515.5 person-years of follow-up observation; each case was observed for a mean of 3.88 yrs following diagnosis. Mean age and mean years of company service of the occupational CTS cases at the time of diagnosis were 46.5 yrs and 22.6 yrs, respectively. Of the 133 cohort members, 130 (97.7%) were male, which is understandable given the predominantly male Navistar factory worker population (96% male) from which the cases arose. Eighty (60.2%) of the occupational CTS claims were for bilateral disease; of the 53 unilateral cases, 39 involved only the right hand and 14 involved only the left. Over the follow-up period, 118 cases underwent surgical release while 15 were managed conservatively. Independent of hand involved, approximately 89% of the cohort underwent surgical release; of the 71 bilateral occupational CTS cases undergoing surgery, 62 (87.3%) had surgery bilaterally.

Lost and Light Duty Days and Claim Cost

The overall mean lost work days, mean light duty days, and mean claim cost for the entire occupational CTS cohort were 71.4 days, 7.6 days, and $13329. Median lost work days for the entire cohort was 54.0 days and 75% of cases accumulated less than 90 lost work days. Table 1 summarizes the lost and light duty days and claim cost for the occupational CTS cohort, stratified by hand involvement and type of treatment. Mean lost work days for surgically treated cases of unilateral and bilateral disease were 58.1 d and 94.9 d, respectively. Conservatively treated cases had low mean lost work days; only one of the fifteen conservative cases involved lost work time. There was very strong evidence (p=0.000) for differences in mean lost work days among the hand involvement and treatment status groups. As the greatest Worker's Compensation cost is typically the payment of temporary total disability, mean claim costs for each category were also significantly different and tracked with lost work days.

The data were also analyzed with stratification upon total number of surgical releases on the claim. These data are summarized in Table 2. Both lost work days and claim cost increased with increasing number of carpal tunnel releases (CTR) per claim and the differences in means for both variables were statistically significant.

Work and Restriction Status

Return to work status following treatment (date of diagnosis for conservatively treated cases and date of return to work (RTW) after last initial CTR for surgically treated cases) over the follow-up period was assessed for 132 of the 133 cohort members. Table 3 provides a tabulation of return to work status for the overall cohort and by treatment type selected. Overall, 61.4 % of the cohort returned to work without any formal restricted activity of the upper extremities over the follow-up period. Seventy-eight percent of the cohort returned to work with less than or equal to thirty days of formal light duty for the upper extremities over the follow-up period; the corresponding values for conservatively and surgically treated cases were 73.3% and 78.6%, respectively. Self-selection by each employee/case away from aggravating activities would not be reflected in these data and cannot be excluded. Only 3.0% (4/132) of the cohort members reviewed either did not, or failed after attempt to, return to work over the follow-up period. Three of these four cases were post-surgical whereas one was a conservatively treated case. Three of the four cases had additional medical conditions contributing to the on-going disability so that occupational CTS was not the sole cause of disability. Of those cohort members who did return to work, 11.7% required permanent duty restrictions with respect to the upper extremities. A greater percentage of conservatively treated cases than surgical cases received permanent duty restrictions affecting the upper extremities (21.4 % vs. 10.5 %) but this difference was not statistically significant (p<0.21).

Outcome for "Standard" vs. "Non-standard" Treatment

Considering one surgical release per affected hand as the standard surgical treatment, it was noted if any lost work days occurred following return to work for the last initial CTR. Of the 118 surgically treated cases, 14 experienced lost work days after return to work from their last initial CTR. The mean lost work days for this group was 200.7 days whereas for those only requiring standard treatment the mean lost work days was 64.2 days (p=0.000). Correspondingly, the mean claim cost for each group was $31091.1 and $12727.8 (p=0.000). Of the 15 cases treated conservatively, only one case involved lost work days; the other fourteen electing conservative treatment were non-lost time cases. The lost work days and claim cost of the one lost-time conservatively treated case were 33 and $7523.0; comparatively, the mean lost work days and mean claim cost of the 14 non-lost-time cases were 0 and $407.4.

Similarly, 8 of the 118 surgically treated cases required more than one CTR on at least one side. There was no difference between unilateral and bilateral cases with respect to the proportion requiring more than one CTR/side (2/47 vs. 6/71, p<0.474). There were statistically significant differences in the mean lost work days and mean claim costs for the group requiring more than one CTR/side vs. the group which did not (mean LWD- 237.9 days vs. 68.8 days and mean cost- $32483.8 vs. $13633.4, p=0.000 for both).

CONCLUSIONS AND RECOMMENDATIONS

In summary, for this occupational CTS cohort over the follow-up period, most surgically treated cases were successfully managed with one CTR per affected hand and accumulated lost time only for the initial CTR on each side; the mean lost work days and mean claim cost for such "standard treatment" were 45.9 days and $9077.6 for unilateral release and 80.1 days and $15913.5 for bilateral releases. Most cases were able to return to work following surgery with either no restrictions or short term light duty. The post-operative return to work experience in this cohort (97.4% at mean follow-up of 3.9 years) is more positive than that reported for meat packing and for Washington state where 52% were working 1 year post-op⁴ and 82% of cases were working 3 years post-op⁵, respectively. For the small number of cases in which lost time occurred after standard surgical treatment and/or more than one CTR/side was performed, mean lost work days and mean claim cost were significantly greater. In general, the conservatively treated cases did not accumulate lost work time, involved low cost, and more frequently required formal, permanent upper extremity restrictions.

Shortcomings of these data sources include no way to capture the role persistent hand symptomatology may play in retirement decisions for those cohort members with the necessary age and service, no way to measure self-selection away from jobs which may aggravate persistent hand symptomatology, and no way to assess persistent hand symptomatology which does not interfere with employee placement (employee performs activities with toleration of any hand symptoms which occur).

Little information exists regarding long term functional outcomes for either conservative or surgical management of occupational CTS. No studies to date have directly assessed on-going symptomatology or factors which may be predictive of long-term poor outcome with this condition. No objective data are currently available regarding expected recurrence rates or symptomatology patterns following treatment for occupational CTS. Therefore, direct assessment of this cohort through a symptom survey, electrophysiologic screening, and anthropometric and physical examinations could provide insight into these issues and represents a natural follow-on to this effort.

REFERENCES

Table 1. Mean lost and light duty days and claim cost for the Navistar Springfield Operations Occupational CTS cohort.

Table 2. Mean lost and light duty days and claim cost for the Navistar Springfield Operations Occupational CTS cohort as a function of number of surgical releases per claim.

Table 3. Working and restriction status with respect to upper extremities of the Navistar Springfield Operations Occupational CTS cohort over the follow-up period.