Prediction equations for permanent impairment of the upper extremity due to the loss of range of motion

Abstract

BACKGROUND:

Musculoskeletal disorders of the upper extremity are one of the major causes of human suffering, increased workers’ compensation expenses, and lost work time, productivity and quality at workplaces. In the developed countries, in case of permanent impairment, a rating is required by legal institutions in order to determine the compensation level. In addition, knowledge of the impairments is a valid reason for companies to take action in order to prevent the advancement and occurrence of the impairments in the future.

OBJECTIVE:

Developing impairment prediction models of the upper extremity due to the loss of range of motion of the wrist, forearm/elbow and shoulder joints.

METHODS:

The relationships between the loss of range of motion and corresponding impairment ratings of upper extremity joints were investigated by the least squares line fitting approach based on the available impairment data.

RESULTS:

Fourteen impairment prediction models due to the loss of range of motion of wrist, forearm/elbow and shoulder joints were developed. Most of the developed prediction models were linear models and several of them were 2nd and 3rd degree polynomial models. The models, overall, had relatively high predictive capabilities (for 10 of the 14 models: R²_adj>0.95; and for the remaining four models: 0.815 ≤ R²_adj ≤ 0.93).

CONCLUSIONS:

The developed equations automated by a simple code or a spreadsheet can easily be used for the evaluation of permanent upper extremity impairment and its effect on permanent impairment of the whole body by physicians, occupational therapists, ergonomists, insurance companies and legal institutions.

Keywords

Work disability musculoskeletal disorders workers’ compensation

1 Introduction

At work or leisure today many people are suffering from the temporary or permanent effects of physical impairments of the hand and upper extremity and other parts of the body. These musculoskeletal disorders (MSDs), some of them resulting from cumulative trauma, cause reduction in productivity, and high medical and compensation expenses, in addition to human suffering [1 –5].

Workers’ compensation laws in a number of developed countries (e.g., USA, Canada and Australia) provide payments for medical expenses, loss of wages and impairment. The workers with injuries do not have to sue the employer; however, having lost the right to sue, the workers’ compensation system assures impairment benefits to all workers who are covered under the law and who meet the criteria for benefits. In case of partial impairment, the law requires a determination of the loss rate for compensation [6 –8].

Therefore, accurate evaluation of the patients’ temporary or permanent anatomic and functional impairment, along with their local and general condition resulting from these deficiencies, is needed [6 , 9–14]. Complete and correct records of patients will help the physicians or other qualified professionals to choose the suitable evaluation and treatment method. Law courts, insurance companies and other judicial institutions are often required to evaluate the trauma and disease results to the hand and upper extremity. Standard methods are necessary for anatomic, functional and cosmetic deficits for accuracy and consistency of the complete impairment evaluations [6 –12].

The correct impairment evaluation of a limb with an injury presumes information regarding its normal functioning. Normally, loss of limitation, structure, motion, strength, pain, and/or loss of sensibility are determined by comparing with the opposite, uninjured limb. If both limbs are impaired, an average limb is considered as the baseline [6 , 12].

An impairment is a significant deviation, loss or loss of use of any body part, system, or function in a person with a health condition, disorder or disease [6]. Example impairments are weak grips or segments with reduced mobility. A medical impairment can develop from an illness or injury. An impairment is considered permanent when it has reached maximal medical improvement, meaning that it is well stabilized and unlikely to change substantially in the next year with or without medical treatment [6]. An impairment may lead to functional limitations or the inability to perform daily activities [6 , 12].

Impairment ratings (percentages) are consensus-derived percentage estimates of loss of activity reflecting severity for a given health condition and the degree of associated limitations in terms of activities of daily living, excluding work. Impairment ratings were designed to reflect functional limitations and not disability [6 , 12].

Work is not included in the clinical judgment for impairment ratings for several reasons: (i) work involves many simple and complex activities; (ii) work is highly individualized, making generalizations inaccurate; (iii) impairment ratings are unchanged for stable conditions, but work and occupations change; and (iv) impairments interact with other factors such as the worker’s age, education and prior work experience to determine the extent of work disability [6, 16]. For example, a person who receives a 20% whole person impairment due to pericardial heart disease is considered from a clinical standpoint to have a 20% reduction in general functioning, as represented by a decrease in the ability to perform activities of daily living. For individuals who work in sedentary jobs, there may be no decline in their work ability although their overall functioning is decreased. Thus, a 20% impairment rating does not correspond to a 20% reduction in work capability [6].

As a result, impairment ratings are not intended for use as direct determinants of work disability. When a physician or other qualified professionals are asked to evaluate work-related disability, it is appropriate for the evaluator, who is knowledgeable about the work activities of the patient to discuss the specific activities the worker can and cannot do, given the permanent impairment [6 , 16].

Disability refers to the effect of the impairment on the ability to perform a job or specific task. In other words, it is the alteration of a person’s capacity to meet personal, social, or occupational demands or statutory or regulatory requirements because of impairment [6–9 , 15].

Permanent disability occurs when the degree of capacity becomes static or well stabilized and is not likely to increase in spite of continuing medical or rehabilitative measures. [6]. Disability may be caused by medical impairment or by non-medical factors. Permanent impairment is a contributing factor to, but not necessarily an indication of, the extent of a patient’s permanent disability. An impaired individual is not necessarily disabled. Impairment becomes disability only if the medical condition limits the person’s ability to meet the requirements that relate to non-medical areas and activities. However, if the individual can meet the demands of a particular task, although impaired, the person is not disabled with respect to those requirements. If adaptations can be made to the work demands or environment, the person may not be disabled from performing that activity [6, 15]. An individual with a medical impairment may not have a disability for some occupations, yet be very disabled for others. That is, impairment assessment is a necessary first step for determining disability, but it is only one of several determinants of disablement [6 , 15]. For example, an individual with repeated hernias and repairs may no longer be able to lift more than 20 kg but could work in a plant where mechanical lifts are available.

In order to get an accurate state of the patient, a combination of anatomic, cosmetic and functional evaluations of the upper extremity is necessary [6]. However, due to the lack of the necessary level of standardization of cosmetic and functional evaluations in precision and reproducibility, the anatomic impairment evaluation forms the basis for upper extremity impairment assessment. A complete anatomic evaluation should include measurement of range of motion of individual joints, strength, muscle testing, sensory evaluation and assessment of pain [6].

The American Medical Association’s (AMA) “Guides to the Evaluation of Permanent Impairment” [6] is the most commonly used reference for evaluating and rating an individual’s permanent impairment in the United States and, increasingly, other countries. The Guides has developed a system for evaluating physical impairment of the whole body due to amputation, sensory loss, abnormal motion, and ankylosis, in addition to a number of other areas of permanent impairment (e.g., sensory systems, mental and behavioral disorders, cardiovascular system, etc.).

However, there are a number of potential shortcomings related to the use of the Guides by the practitioners: First, the impairment rating charts consist of impairment rates corresponding to multiples of 5 or 10 joint angles only. Hence, for other joint angles, the users are required to either interpolate or round to the nearest angle value. In addition, the use of the charts requires training and manual calculations of impairment rates, and thus, use of them is time consuming.

Therefore, to overcome these shortcomings, the aim of this study was to develop prediction models of permanent impairment of the upper extremity due to the loss of range of motion of the wrist, elbow and shoulder joints.

2 Methods

2.1 Overview

In developing the prediction models for permanent impairment of upper extremity due to the loss of joint range of motion, and for the impairment calculations of whole extremity and whole body, the impairment data and guidelines provided in American Medical Association’s “Guides to the Evaluation of Permanent Impairment” [6] were used. The upper extremity is divided into four regions for impairment evaluation purposes: hand/digits, wrist, elbow/forearm, and shoulder. An impairment evaluation is a medical evaluation performed by a physician, using a standard method as outlined in the Guides, to determine permanent impairment associated with a medical condition. An impairment evaluation may include a quantitative impairment rating by a treating or non-treating physician. The impairment value of the segments of the extremity may be calculated in terms of impairment to the total extremity and from that to the body. A certain percentage factor for impairment may be given to defects such as awkwardness, incapacity and disturbance of function, resulting from the total loss of function of one of the extremity segments. Evaluation of impairment of the upper extremity joints is reflected in loss of motion or ankylosis [6]. However, the models developed in the present study did not consider the amputation or ankylosis cases, but only partial loss of motion of the upper extremity joints (excluding the digits and hand). This was because in industry, partial motion loss is more common.

More specifically, this study considered the development of wrist, elbow/forearm and shoulder joint impairment models due to the loss of range ofmotion.

2.2 Whole body approach to impairment ratings

The Guides’ impairment ratings reflect the severity of the organ or body system impairment and the resulting limitations of the whole person. In some musculoskeletal regions, a hierarchy of various values, from distal to proximal, is used to reflect the relevant importance of certain parts in each region. The various hierarchical relationships of the musculoskeletal system are exemplified in Fig. 1. As the figure indicates, the total upper extremity impairment (100% ) corresponds to 60% and total lower extremity impairment corresponds to 40% of whole body impairment.

2.3 Upper extremity impairment calculations according to the guide

In the use of the Guides, physicians or other users measure the maximum active wrist, forearm/elbow, and shoulder range of motions (ROMs) and record the actual goniometer readings. Then, they match the angles with their corresponding impairment ratings presented as charts in [6], which are tabulated altogether in Table 1. The impairment values for the angles falling between those listed in the table may be adjusted or interpolated proportionally in the corresponding interval.

2.3.1 Wrist motion impairment

The wrist functional unit represents 60% of upper extremity function. The wrist has two functional units of motion, each contributing a relative value to its function. The unit-of-motion impairments are converted to upper extremity impairments by multiplying their respective values by 60% as follows [6]:

Flexion and extension unit: 70% of wrist function: 70% ×60% = 42% of upper extremity function.

Radial and ulnar deviation unit: 30% of wrist function: 30% ×60% = 18% of upper extremity function.

The normal ROM for flexion-extension and radial-ulnar deviation are from 60 flexion to 60° extension, and 20 radial deviation to 30 ulnar deviation, respectively. Positions of function are 10 flexion to 10 extension and from neutral to 10 ulnar deviation [6].

2.3.2 Forearm/elbow motion impairment

The elbow functional unit represents 70% of upper extremity function. The elbow joint has two functional units of motion, each contributing a relative value to its function. The unit-of-motion impairments are converted to upper extremity impairments by multiplying their respective values by 70% as follows [6]:

Flexion and extension: 60% of elbow function: 60% ×70% = 42% of upper extremity function.

Pronation and supination: 40% of elbow function: 40% ×70% = 28% of upper extremity function.

The normal elbow ROM for flexion-extension and forearm pronation-supination are from 0 to 60 flexion and 0 to 80 extension in the sagittal plane; and from 0 to 80 supination and from 0 to 80 pronation. Here, for the purpose of this study, 80 extension is assumed as the reference 0. Position of function is 0 which corresponds to 80 extension [6].

2.3.3 Shoulder motion impairment

The shoulder functional unit represents 60% of upper extremity function. The shoulder has three functional units of motion, each contributing a relative value to its function. The unit-of-motion impairments are converted to upper extremity impairments by multiplying their respective values by 60% as follows [6]:

(1) Flexion: 40% of shoulder function.

Extension: 10% of shoulder function.

Flexion and extension unit: 50% of shoulder function or 50% ×60% = 30% of upper extremity function.

(2) Abduction: 20% of shoulder function.

Adduction: 10% of shoulder function.

Abduction and adduction unit: 30% of shoulder function, or 30% ×60% = 18% of upper extremity function.

(3) Internal rotation: 10% of shoulder function.

External rotation: 10% of shoulder function.

Internal and external rotation unit: 20% of shoulder function, or 20% ×60% = 12% of upper extremity function.

The normal ROM for shoulder flexion-extension, abduction-adduction and internal-external rotation are from 0 to 180 flexion, and 0 to 50 extension in the sagittal plane; from 0 to 180 abduction and 0 to 50 adduction, and from 0 to 90 internal rotation and 0 to 90 external rotation [6].

2.4 Analysis of the impairment data

Using the impairment data reported in the Guides, the relationships between maximum wrist, forearm/elbow and shoulder joint angles and corresponding impairment values were investigated through curve fitting using Minitab statistical analysis software (Minitab 16). As the first step, scatter plots were constructed to visually examine the type of relationship between the joint angles and the corresponding impairment rates. To obtain the best fit lines, the least squares approach was used and 1st, 2nd and 3rd order polynomials were fitted. The lines with the highest explained variance (R²_adj values) were determined as the best fit lines. Figures 2 through 4 depict the obtained best fit lines which show the relationships between the response (impairment rate) and the predictor variable (maximum achievable joint angle) for each of the upper extremity joints. The plots indicate strong negative linear or curvilinear relationships for all associated pairs of (predictor, response) variables. The obtained models are documented in the results section. These models estimate the impairment rates as a function of loss of wrist, forearm/elbow and shoulder ROM angles.

3 Results

The developed 14 impairment equations are presented in Table 2. Three of the four impairment prediction models of the wrist (flexion, extension and radial deviation) were linear, whereas ulnar deviation was a 2nd degree polynomial model. Elbow flexion and extension models were also 2nd degree polynomial models. On the other hand, forearm pronation has a 3rd degree polynomial and supination has a linear model. Four of the shoulder impairment models (extension, adduction and external and internal rotations) are linear, whereas the flexion and abduction impairment models are 3rd degree polynomialmodels.

With the exception of shoulder adduction and external rotation, the corresponding R²_adj values were relatively high (>0.9) for all the models. That is, for almost all the models, more than 90% of the variability in the response variable (impairment) can be explained by the prediction models developed.

The details of the estimations of whole upper extremity and whole person impairments by using the developed equations are presented in the following subsections (3.1–3.4).

3.1 Wrist impairment models

Using the developed wrist impairment models (Table 2), the impairment ratings (or percentages) can be calculated by following the steps below:

Measure the maximum active wrist flexion-extension and radial-ulnar deviation angles and then record the actual goniometer readings.

Enter the angle values to the developed wrist impairment models (for flexion, extension, radial deviation and ulnar deviation).

Add IW_F% and IW_E% to obtain the percent of upper extremity impairment contributed by decreased wrist flexion and extension.

Add IW_UD% and IW_RD% to obtain the percent of upper extremity impairment value contributed by decreased wrist lateral deviation.

Then the impairment of the upper extremity and whole body due to the abnormal motion at the wrist, respectively, can be estimated using the Equations (1) and (2) as follows:

Impairment rating of the upper extremity due to the wrist loss of motion: ${IUE}_{w} (%) = ({IW}_{F} % + {IW}_{E} %) + ({IW}_{RD} % + {IW}_{UD} %)$ (1)

Impairment of the whole person due to the wrist loss of motion: ${IP}_{W} (%) = 0 . 60^{*} {IUE}_{W}$ (2)

3.2 Forearm/elbow impairment models

Using the developed forearm/elbow impairment models (Table 2), the impairment ratings (or percentages) can be calculated by following the steps below:

Measure the maximum active elbow flexion-extension and forearm pronation-supination angles and record the actual goniometer readings.

Enter the angle values to the developed elbow impairment models (for flexion, extension, pronation and supination).

Add IE_F% and IE_E% to obtain the percent of upper extremity impairment contributed by decreased elbow flexion and extension.

Add IF_P% and IF_S% to obtain the percent of upper extremity impairment value contributed by decreased forearm rotation.

Then impairment of the upper extremity and whole body due to the abnormal motion at the forearm/elbow, respectively can be estimated using the Equations (3) and (4) as follows:

Impairment rating of the upper extremity due to the forearm/elbow loss of motion: ${IUE}_{FE} (%) = ({IE}_{F} % + {IE}_{E} %) + ({IF}_{P} % + {IF}_{S} %)$ (3)Impairment of the whole person due to the forearm/elbow loss of motion: ${IP}_{FE} (%) = 0 . 60^{*} {IUE}_{FE}$ (4)

3.3 Shoulder impairment models

Using the developed shoulder impairment models (Table 2), the impairment ratings (or percentages) can be calculated by following the steps below:

Measure the maximum active shoulder flexion-extension, abduction-adduction, and external-internal rotation angles (in degrees) and record the actual goniometer readings.

Enter the angle values to the developed shoulder impairment models (for flexion, extension, abduction, adduction, external rotation and internal rotation)

Add IS_F% and IS_E% to obtain the percent of upper extremity impairment contributed by decreased shoulder flexion and extension.

Add IS_AB% and IS_AD% to obtain the percent of upper extremity impairment value contributed by decreased shoulder abduction and adduction.

Add IS_ER% and IS_IR% to obtain the percent of upper extremity impairment value contributed by decreased shoulder rotation.

Then the impairment of the upper extremity and whole body due to the abnormal motion at the shoulder, respectively can be estimated using the Equations (5) and (6) as follows:

Impairment rating of the upper extremity due to the shoulder loss of motion: ${IUE}_{S} (%) = ({IS}_{F} % + {IS}_{E} %) + ({IS}_{AB} %$ (5) $+ {IS}_{AD} %) + ({IS}_{ER} % + {IS}_{IR} %)$

Impairment of the whole person due to the shoulder loss of motion: ${IP}_{S} (%) = 0 . 60^{*} {IUE}_{S}$ (6)

3.4 Combining impairment ratings

For multiple regional impairments, such as those of the wrist, forearm/elbow and shoulder, the impairment of each region is first expressed individually as upper extremity impairments and then combined to determine the total upper extremity impairment. The latter is finally converted to whole person impairment. The combined value determination is based on the following formula, where A, B, and C are regions of the upper extremity [6]:

A% +B% (100% − A% ) = I_AB% (the combined value of A% and B% ).

To combine a third region to the combined value of I_AB% , the procedure is the same:

I_AB% +C% (100% - I_AB% ) = I_ABC% (the combined value of A% , B% and C% ).

This process can be repeated indefinitely.

Example: Assume the following are computed after examining a patient: IU_w = 15% , IU_FE = 20% , and IU_S = 30% .

(1) The combined total impairment of the upper extremity due to the loss of wrist, elbow, and shoulder motion is calculated as follows:

IU_W - FE = 15% +(100% –15% )^*20% = 32% (the combined upper extremity impairment due to the loss of wrist and forearm/elbow motions)

IU_{W - FE - S} = 32% +(100% –32% )^*30% = 52% (the total combined upper extremity impairment due to the loss of wrist, forearm/elbow and shoulder motions)

(2) In this case, impairment of the whole body is:

IP = 60% ^*52% = 31% .

3.5 Comparisons of predicted and chart impairment values

The impairment values (% ) were calculated using the prediction equations to be compared to the chart values. The absolute differences (in percentage points) between the predicted and chart values corresponding to the maximal possible joint angles are shown in Table 3. The difference values for all the impairment ratings are <0.5, except IS_F and IS_AB cases for which the values are >1 for a number of joint angles. These results, overall, indicate that the developed equations have high prediction capabilities and thus the differences would be negligible for most of the impairment calculations. (Note: the performances of paired t-tests to determine the statistical significance of the differences between the predicted and chart values were not possible since the standard deviations and sample sizes of the impairment data were not obtainable from the Guides).

4 Discussion

A total of fourteen impairment prediction models of the upper extremity were developed corresponding to wrist, elbow/forearm and shoulder joint loss of motions. Generally speaking, the predictive capabilities of the wrist and forearm/elbow models are very high as indicated by the R²_adj values. The absolute differences (in percentage points) between the predicted and chart values in the guides (<0.5 for the most joint angles) further support this result.

The shoulder impairment models also have high prediction capabilities, except the adduction and external rotation models, which have relatively low R²_adj values compared to the other models. Although the impairment models of adduction and external rotation do not have strong prediction capabilities, their contributions to the total impairment of upper extremity is not as significant as the other shoulder motions. Indeed, the impairment rating percentage for both motions at the highest level is only 2% . For the case of the abduction model, although it has high R²_adj value, the comparison results indicate that for abduction angle ≥110, the predictive capability of the model is relativelypoor.

Hence, the users of the impairment prediction equations developed through this study should take these points into account when calculating the impairment rates.

Overall, the developed prediction models can be used with relatively high accuracy to estimate the total impairment of the upper extremity due to the wrist, forearm/elbow and shoulder motion impairments and in relation to the impairment of the whole body.

The advantages of the use of the prediction equations over the chart in the guides can be summarized as follows. The existing charts consist of impairment rates for multiples of 5 or 10 joint angles only. Thus, the use of the charts requires interpolation or rounding to the nearest impairment value for the measured joint angle. This issue will be solved by the use of the prediction equations. The charts are confusing, time consuming and require hand calculations and training; on the other hand, with coding or use of a spreadsheet, this issue will be minimized by the prediction equations. It will also be easier to document the results with the software that will be written for the purpose.

As with most studies, this study also has some limitations. Firstly, the study considered impairment evaluations of the upper extremity due to the loss of motion only. A complete anatomic evaluation of the upper extremity should also include strength, muscle testing, sensory evaluation and assessment of pain. Secondly, the motion impairments of the hand/digits and the ankylosis cases were not considered in this study.

5 Conclusions

The available upper extremity impairment data in the Guides were analyzed and impairment prediction models were developed to simplify the process for users. This study indicates that the formulation of impairment predictions is possible for regions of the upper extremity, based on the existing impairment data. The prediction equations developed through this study replace the use of time consuming and confusing charts. It is expected that by writing a simple code or using a spreadsheet to automate the process, the models will be very useful and practical for physicians, occupational therapists, ergonomists, insurance companies and legal institutions for estimating the upper extremity impairment due to the loss of range of motion and its contribution to whole body impairment.

Footnotes

Acknowledgments

This study is supported by Boğaziçi University Scientific Research Projects (BAP) Fund (Project no. 06HA302).

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