Device-assisted muscle strengthening in the rehabilitation of patients after surgically stabilized vertebral fractures

Article Outline

• Abstract
• Methods
  • Study procedure
  • Pain history
  • Strength measurement
  • MRI examination
  • Statistical evaluation
• Results
  • Qualitative pain assessment
  • Quantitative pain assessment
  • Strength measurement
  • Magnetic resonance imaging
  • Strength: muscle CSA relationship
  • Strength measurement: pain
• Discussion
  • Group 1: pain improved, strength increase
  • Group 2: pain unchanged, strength improved
  • Group 3: pain worse, strength slightly improved
• Conclusions
• References
• Copyright

Abstract 

Kramer M, Dehner C, Katzmaier P, Neuwirth F, Ebert V, Elbel M, Hartwig E. Device-assisted muscle strengthening in the rehabilitation of patients after surgically stabilized vertebral fractures.

Objectives

To investigate the effects of a device-assisted muscle strengthening exercise program on the surgically traumatized multifidus musculature and on the intact longissimus and iliocostal muscles and to assess the possible relationship between patients’ reported pain symptoms and changes in muscle strength or changes in cross-sectional area (CSA) of the musculature.

Design

Open, prospective noncontrolled prepost intervention study.

Setting

University-affiliated center for ambulant physiotherapy.

Participants

Fifteen patients who had undergone dorsal osteosynthesis for treatment of thoracolumbar vertebral fracture and who complained of persistent back pain.

Intervention

Twelve-week device-assisted training exercise program.

Main outcome measures

Patients’ pain score, muscle strength, and the CSA of the paravertebral musculature determined by magnetic resonance imaging (MRI) were assessed before and after the exercise program.

Results

MRI findings revealed no increase in the CSA of the multifidus muscle in any patient (median change, −.27cm²). All patients, however, exhibited hypertrophy of both the longissimus and iliocostal muscles (median change, 1.39cm²). Significant increase in muscle strength was observed in 14 of 16 patients (median increase, 56%; range, 0.7%−126.4%). The median overall pain score improved from 19 (range, 7–24) to 16 (range, 5–27). The change in muscle strength and muscle CSA, however, showed no correlation. There was also no correlation between increase in muscle strength and changes in pain scores.

Conclusions

The device-assisted training program resulted in hypertrophy of the iliocostal and longissimus muscles and an increase in muscle strength in patients with surgically stabilized vertebral fractures. About half of the patients reported relief of pain. No correlation was found between hypertrophy, increase in muscle strength, and relief of pain. The surgically damaged multifidus musculature, however, did not show any change in CSA and was not accessible to rehabilitative measures.

Key words:  Low back pain , Muscles , Rehabilitation

MUSCULAR INSUFFICIENCY has been discussed as a frequent cause of chronic back pain.¹ Magnetic resonance imaging (MRI) of patients with chronic pain (symptoms duration, >18mo) showed that the volume of the lumbar musculature is significantly decreased in comparison with persons in the acute pain stage.² Further studies have shown that chronic back pain is associated with disturbances of proprioceptive sensation,³ reduced muscle strength,⁴ increased muscular fatigability,⁵ and changes in muscular structure.⁶

These findings have given impetus to the development of exercise programs aimed at increasing the strength of the paravertebral musculature. The objective of these programs was to achieve an increase in muscular strength through hypertrophy of the affected muscles, with a theoretical resultant reduction in patients’ back pain. Device-assisted exercise programs produced increased strength of the back muscles both in healthy subjects⁷ and in patients with chronic back pain.⁸ The exact relationship between increased muscular strength and hypertrophy of the paravertebral musculature, however, has yet to be sufficiently elucidated.⁹, ¹⁰ Increased muscular strength has been ascribed primarily to improvements in the activation and coordination of the back muscles, which are the first changes that occur in the early phase of the exercise program.¹¹ Significant muscular hypertrophy, however, can be expected only after a training period of about 5 weeks.¹²

Exercise programs that increase the strength of the back musculature should aim at a reduction in chronic back pain. Studies to date have concentrated on patients with lower back pain of nonspecific causation.¹³ One study¹⁴ showed that strength exercises targeting the back musculature were associated with an improvement in back pain. Similarly, device-assisted muscle strength-building exercise programs led to improvements in pain symptoms in comparable patient cohorts.⁸ Data on the efficacy of device-assisted programs in postsurgical patients, however, have not been published. Surgical patients differ from previously studied groups because of the muscle damage occasioned by the surgical approach.¹⁵, ¹⁶, ¹⁷

Against this background and because of the steadily increasing number of patients undergoing spinal surgery, the question of the extent to which patients with chronic back pain after major surgical procedures involving the vertebral column can be rehabilitated takes on great relevance. Underlying this question is another: is muscular development even possible in the vertebral segments that have been traumatized by the surgical approach?

The objective of the present study was to assess the effects of a device-assisted muscle strength-building exercise program on the paravertebral musculature, including the surgically traumatized multifidus muscle and the intact longissimus and iliocostal muscles. Further, we assessed the possible relation between patients’ reported pain symptoms and changes in muscle strength or in changes of the cross-sectional area (CSA) of the musculature.

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Methods 

The study conformed to the Declaration of Helsinki. All patients gave written consent. Included in this study were 15 patients (9 men, 6 women) who had undergone temporary dorsal osteosynthesis with placement of an internal fixation (USS Fracture System^a) device for treatment of a fracture in the thoracolumbar spinal column (T12, 2 patients; L1, 9 patients; L2, 3 patients; L3, 1 patient). Median age at the time of treatment was 36 years (range, 18–57y). Six patients exhibited neurologic symptoms, including sensory disturbances in segment L5 in 2 patients, motor deficits in segments L3 or L4 in 2 patients, and combined sensory and motor deficits (segments L3 or L5) in another 2 patients. The internal fixation device was removed from all patients within 6 months. Median follow-up from device removal to initiation of exercise training was 13 months (range, 6–24mo). All patients reported overwhelming residual complaints and restriction of motion despite exhaustion of conservative methods, including intensive physiotherapy (PT).

Study procedure 

Before beginning the exercise phase, patients underwent radiography of the thoracolumbar spine to exclude bony problems that prohibit heavy loading of the spine. Strength of the paravertebral musculature was measured and each patient also had an MRI examination to determine the CSA of the paravertebral muscles.

The training was done on David exercise devices,^b which are designed for training of the paraspinal muscles. By fixing the pelvis, the function of the gluteal muscles can be eliminated, thus allowing a specific exertion of the back muscles. Exercises were performed in extension (device 110), flexion (device 130), axial lumbar rotation to both sides (device 120), and lateral bending to both sides (device 150).

During the first 2 weeks, patients were instructed in the correct execution of the training program using the exercise devices without application of weights. After this familiarization phase, we again assessed the maximal voluntary contraction (MVC).

During the next training phase, exercises were conducted with weights corresponding to 30% of patients’ maximum strength. Each exercise was to be repeated between 30 and 40 times. Because of patients’ pain-related reduced capacity, exercises requiring numerous repetitions were performed with less applied weight. The program’s intent was that muscular development should occur in all directions of normal spinal motion (flexion, extension, lateral flexion, rotation). The exercise plan encompassed 2 training sessions per week, which all patients attended jointly. After 4 weeks, we increased the applied weight, after taking a new MVC measurement, to 45% of maximum strength with a simultaneous reduction in repetition of exercises to 20 to 30. After another 4 weeks, patients reached the final level of 60% of MVC and 10 to 20 repetitions per exercise. A final assessment of all parameters was conducted at week 12, the end of the exercise phase.

Pain history 

The pain history was a qualitative assessment of the intensity, duration, frequency, and extent of patients’ reported pain. Patients were to differentiate between increasing, decreasing, and constant severity of symptoms.

Semiquantitative assessment of pain intensity and frequency was based on a visual analog scale (VAS) of 0 to 10 points. A total score (range, 0–30) is calculated from the respective values assigned for maximum pain intensity (range, 0–10), mean pain intensity (range, 0–10), and pain frequency (range, 0–10). The progression in pain symptoms was evaluated according to the following criteria: improvement (overall score reduced by at least 5 points), no change (overall score changed by less than 5 points in either direction), worsening (overall score increased by at least 5 points). Pain data were documented at intake, after 4 weeks, and at 12 weeks completion of therapy.

Strength measurement 

Measurement of isometric MVC was conducted before beginning therapy and again at 4 and 8 weeks and after completion of therapy (at 12wk). Patients were placed in a position of 30° of flexion and asked to press against the immobilized lever arm. The determination of MVC of the torso musculature was performed by using a David training device. MVC was determined separately for flexion, extension, lateral flexion, and left and right rotation. The average of these 4 strength measurements was used as the total strength.

MRI examination 

Twelve patients underwent MRI (transverse slices) before beginning the exercise program. In 3 cases, patients’ claustrophobia precluded successful MRI. For determination of muscle CSA and evaluation of potential increase in muscle mass, these 12 patients underwent a second MRI examination of the vertebral column after completion of the exercise phase. The MRI was performed in the same segments and the same axial cuts were analyzed before and after training.

MRI visualized the lumbar musculature (fig 1), which is comprised of the more medial multifidus musculature and the more lateral erector spinae musculature (the iliocostal and longissimus muscles). Measurement of the CSA consisted of size determination of the lumbar musculature at the level of the vertebral pedicle of the fractured vertebra and above or below the fracture at the level of the dorsal stabilization. Every structure was marked 3 times manually by an examiner while the CSA and its average value were calculated by computer (fig 2). Scar tissue was seen in all patients inside the multifidus muscle. It would have been impossible to mark only muscle without scar tissue; therefore the multifidus was marked with the scar tissue inside. The interpretation was blinded with respect to time (before or after the training program).

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• Fig 1. 

  MRI for determination of muscle volume before training. *Multifidus muscle; †iliocostal and longissimus muscles.

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• Fig 2. 

  Comparison of the 2 muscle areas before (dark) and after (light) training. The area of the multifidus muscle remained unchanged whereas the area of the iliocostal and longissimus muscles increased. *Multifidus; †iliocostal and longissimus.

Statistical evaluation 

We used the Wilcoxon test for dependent samples to determine the statistical significance of the parameters studied. Statistical significance was defined as values of P less than .05.

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Results 

Qualitative pain assessment 

Table 1 presents the findings of the qualitative pain assessment. In comparison with findings returned at 4 weeks, there was significant improvement in all measured pain characteristics after 12 weeks. Only 1 patient reported worsening in terms of the duration of pain after 12 weeks. In 9 of 15 (66.6%) patients, there was improvement in at least 2 areas of pain quality; 4 of these patients reported improvement in all areas of pain quality. Patients benefited most clearly in the areas of pain duration and extent, with 8 patients each reporting improvement in these areas.

Table 1. Qualitative Reports of Pain Intensity, Duration, Frequency, and Extent After 4 and 12 Weeks of Exercise

Reports	Improvement		No Change		Worsening
After 4wk
Intensity	2	13.33	12	80.00	1	6.67
Duration	1	6.67	14	93.33	0	0.00
Frequency	2	13.33	8	53.33	5	33.33
Extent	0	0.00	13	86.67	2	13.33
After 12wk
Intensity	6	40.00	9	60.00	0	0.00
Duration	8	53.33	6	40.00	1	6.67
Frequency	7	46.67	8	53.33	0	0.00
Extent	8	53.33	7	46.67	0	0.00

NOTE. Values are number and percentage of the patients in each category.

Quantitative pain assessment 

Median overall score before therapy was 19 (range, 7–24; table 2). After completion of the exercise program (12wk), the median score was 16 (range, 5–27). On completion of the exercise program, 5 patients reported improvement in pain; in 7 patients, there was no change; and in 3 patients, there was worsening of pain symptoms (see table 2).

Table 2. Pain Score Derived from Scores for Maximum Pain Intensity, Median Pain Intensity, and Pain Frequency

NOTE. VAS scales for each pain component ranged from 0 to 10; therefore, the scale for the overall pain score ranged from 0 to 30.

Strength measurement 

Fourteen of 16 (87.5%) patients exhibited increased strength in all directions of motion of the lumbar spine. In 1 patient, an increase in strength could only be produced in lateral flexion, with simultaneous reduction in strength in other directions of motion. The median percentage of improvement in total strength was 56% (range, 0.7%–126.4%).

Figure 3 presents percentage increase in strength in the different directions of motion at completion of the exercise program. The median increase in strength for right lateral flexion was 72.0% (range, 6.3%–178.8%), and for left lateral flexion, 56.2% (range, 6.1%–250.0%). The least pronounced increase in strength was observed for flexion, at a median 16.9% (range, −1.6% to 76.0%). For extension, the increase was 30.2% (range, −7.4% to 120.7%); for right rotation, 51.0% (range, −7.8% to 302.7%); and for left rotation, 44.2% (range, −14.8% to 175.5%).

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• Fig 3. 

  Increase in muscle strength after completion of the 12-week exercise program. Legend: black square is the median; grey box, the 1st and 3rd quartile; line, the range from minimum to maximum.

Magnetic resonance imaging 

Table 3 shows the CSA of the multifidus muscle, the iliocostal and longissimus muscles, and the total musculature (in percent) before beginning and after completing the 12-week exercise program. In the 12 patients undergoing MRI examinations, a median size increase in the total CSA of 1.03cm² (5.9%; range, −0.16cm² [−0.6%] to 5.1cm² [26.8%]) was observed after completion of the exercise program. For the longissimus and iliocostal muscles, the median increase in CSA was 1.39cm² (8.3%; range, 0.22cm² [0.9%] to 5.22cm² [30.5%]); and for the multifidus musculature, the mean increase in CSA was −0.27cm² (−17.5%; range, −0.03cm² [−1.5%] to −0.84cm² [−45.4%]) (fig 4).

Table 3. CSA of the Multifidus, Iliocostal, and Longissimus Muscles and the Total Musculature Before and After Completion of the 12-Week Exercise Program

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• Fig 4. 

  Change in CSA of the multifidus, the iliocostal and longissimus, and the total musculature after completion of the 12-week exercise program. Legend: black square is the median; grey box, the 1st and 3rd quartile; line, the range from minimum to maximum.

Strength: muscle CSA relationship 

A statistically significant relationship existed between muscle strength and CSA of the longissimus and iliocostal muscles before and after exercise program participation. Before the exercise program, the coefficient of correlation for extension strength was .770 and .745 for average strength (Fig 5, Fig 6). After completion of the exercise program, the corresponding coefficients of correlation were .685 and .794 for extension strength and average strength, respectively (see Fig 5, Fig 6). The change in muscle strength and muscle CSA, however, showed no significant correlation (average strength, .176; extension strength, .406).

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• Fig 5. 

  Relation between average muscle strength and CSA of the longissimus and iliocostal muscles before (•) and after (△) the exercise program.

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• Fig 6. 

  Relation between extension muscle strength and CSA of the longissimus and iliocostal muscles before (•) and after (△) the exercise program.

Strength measurement: pain 

No significant correlation existed between increase in muscle strength and changes in pain scores. In patients (n=5) reporting an improvement in pain, the median total strength increase was 50.8% (range, 39.1%–57.3%). In the group of patients (n=7) with no change in pain symptoms, a median increase in total muscle strength of 61.1% (range, 23.7%–126.4%) was observed. Patients (n=3) whose symptoms worsened showed a median increase in total strength of 26.0% (range, 0.7%–91.2%).

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Discussion 

To date, the published results from therapy studies have been restricted to nonsurgical cohorts.¹⁸ Included in the present study were patients who had undergone dorsal stabilization of vertebral fractures and who had postoperatively developed chronic back pain. These patients differ from samples with degenerative and nonspecific causes of back pain. They have local tissue damage related both to the original trauma and to the muscle damage required by the surgical approach. Studies have reported nerve damage,¹⁶ loss of muscle mass,¹⁵ and structural changes¹⁷ in the musculature as sequelae of surgery.

The surgical damage to the multifidus muscle is confirmed by the findings of the present study. No patients responded to the exercise program with an increase in the CSA of the multifidus muscle. MRI revealed changes in the muscle consistent with scar-tissue formation. Hence, the multifidus does not contribute in any relevant way to the increase in muscular strength recorded in patients undergoing rehabilitation.

Destruction of the multifidus musculature over a not inconsiderable length (3 vertebral segments) raises the question of whether the remaining musculature in this region can sufficiently stabilize the vertebral column and whether targeted exercise can achieve a compensatory development of these muscles.

Increased muscle strength was recorded in all patients completing the exercise program. This was associated with an increase in CSA of the longissimus and iliocostal muscles. Both before and after therapy, there was in all patients a significant correlation between muscle size and the total and extension muscle strength. The change in muscle CSA, however, did not significantly correlate to increase in strength. One explanation for this could be individual variations in strength measurement because of pain or inadequate motivation. Another unknown is whether some patients had achieved a higher degree of activation and coordination from their prestudy PT than had other patients, with the result that these patients could have more rapidly developed muscular hypertrophy. Moreover, no other study of patients with back pain has shown an acceptable correlation between hypertrophy and increased strength.⁹, ¹⁰

Although the therapy program was a success in terms of measured increase in muscle strength and CSA, the question remains whether these parameters correlate with the objective of therapy, which is reduction of pain. Many researchers have attempted to identify correlations between muscle strength and muscle size and patients’ reported pain symptoms. Although Cooper² and Parkkola⁶ and colleagues were able to show that patients with chronic back pain exhibit decreased volume of the paraspinal musculature, other studies failed to identify this correlation.¹⁹ Although patients frequently develop chronic back pain after dorsal stabilization procedures,²⁰ no rehabilitation studies with surgical patient collectives have been published.

The increased strength of the longissimus and iliocostal muscles secondary to hypertrophy of these muscles correlated inadequately with changes in patients’ pain symptoms. We found no significant correlation between increased muscular strength and either qualitative or semiquantitative pain data. The fact that truly objective criteria for the evaluation of pain are not available makes assessment of these factors more difficult. Instead, assessment of pain characteristics must be based on qualitative or semiquantitative data, which cannot be fully reproduced and which, even within the present study, show significant variability. Further, various factors may affect the occurrence and perception of pain and its subjective severity. Personal motivation, psychic stress, and the patient’s social surroundings may limit the efficacy of the exercise program. For example, Indahl et al²¹ found that the general uncertainty of the patient about the severity of his disease is a major problem affecting the treatment of chronic back pain. Bendix et al²² showed that a rehabilitation program’s success can depend on other factors, including age, number of available sick days, and job-related stress.

All previous therapy studies, however, are in agreement that certain patients benefit from device-assisted training, whereas others report no improvement in pain symptoms. In some patients, the improvement in pain symptoms is associated with an increase in strength but not in others. Based on the criteria “pain” (semiquantitative data) and “muscle strength,” the patients in our collective can be divided into 3 groups: (1) improvement in pain associated with increased muscle strength, (2) no change in pain symptoms despite increased muscle strength, and (3) worsening of symptoms with slight increase in muscle strength.

Group 1: pain improved, strength increase 

Five patients experienced improvement in pain symptoms considered relevant as measured by the VAS. These patients also had a 50.8% median increase in total strength as a result of their participation in the exercise program. Hence, muscular weakness from surgical damage and pain-related atrophy of type 2 muscle fiber could have been at least partially responsible for chronic back pain in these patients. A similar correlation between muscle strength and the intensity of pain symptoms has also been described in studies of persons with back pain of nonspecific etiology. Kankaanpää et al²³ found a significant correlation between increased muscle endurance and improvements in pain symptoms in patients participating in a device-assisted exercise program. Similarly, Mooney et al²⁴ found in a study of 8 patients that a device-assisted exercise program is associated with increase in extension strength and improvement in pain symptoms. In another study, Mannion et al²⁵ identified a significant correlation between pain symptoms and maximum isometric muscle strength, although only before therapy. After the 3-month exercise phase, pain symptoms improved in all therapy groups (active PT, device-assisted exercise, aerobics). The findings of the Mannion study, however, failed to show a clear correlation between this improvement and the increased strength.

Group 2: pain unchanged, strength improved 

In this group, participation in the exercise program was also associated with an increase in muscle strength. Although the increase in total strength was more pronounced in this group than in either groups 1 or 3 (median, 61.1%), it was not associated with any relevant improvement in semiquantitative pain data. In this group, other causes of chronic back pain must be considered in addition to traumatic or surgery-related muscle damage. A similar conclusion was reached by Weber et al¹⁷ in a study with patients who had undergone dorsal spondylodesis. In 32 patients who underwent a second operation for removal of the internal fixation device, muscle biopsy specimens were obtained and examined for changes in histologic structure. There was no identifiable correlation between the presence of histologic changes in the muscle and the intensity of chronic back pain. Other causes of pain, which are probably inaccessible to muscle training, would include traumatic damage to the intervertebral disks or static changes in vertebral shape. These changes are identifiable radiologically but also do not correlate adequately to patients’ reported pain symptoms.²⁶ In our sample, the effects of surgery-related structural changes in the musculature on the biomechanical characteristics of the vertebral column must also be considered.²⁷ After surgery, the passively extensible and actively contractile musculature is replaced by scar tissue, which does not share these properties. The cicatricial changes in the musculature may contribute to a decrease in flexion motion in the surgically affected segments and possibly to higher motion amplitudes in the adjacent cranial and caudal segments. At the same time, on extension, the short-length multifidus fibers are supplanted by the movement of the longer lateral musculature. With the exception of the surgically manipulated vertebral segments, the insertion points of the musculature shift from medial to more lateral. The surgically treated segments, therefore, are subject to significantly altered biomechanic conditions, whose contribution to the development of pain remains unknown.

Group 3: pain worse, strength slightly improved 

Three patients experienced clinically relevant worsening in their pain symptoms, despite an increase in strength (median, 26%), which, however, was significantly smaller than that observed in patients in the other groups. Two of these patients had exhibited motor deficits before beginning therapy, confirming the preexistence of damage to neural structures. These patients’ participation in an active exercise program may have resulted in further irritation to the nerves, with subsequent increase in pain intensity. It is also conceivable that a 26% increase in strength is not sufficient to improve pain symptoms. Because of the smaller increase in strength in this group, it also appears questionable whether each patient possessed the personal characteristics and adequate motivation to participate in the training program up to the limits of his/her pain tolerance.

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Conclusions 

About 50% of patients experienced improvement in semiquantitative and qualitative pain symptoms after participation in a device-assisted muscle strength-building exercise program. The program resulted in an increase of strength accompanied by hypertrophy of the iliocostal and longissimus muscles. Nevertheless, no correlation was found between posttreatment strength measurement, achieved muscle hypertrophy, and pain relief.

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• a Synthes, Eimattstr 3, 4436 Oberdorf, Switzerland.
• b David, Nelsonallee 22, 89233 Neu-Ulm, Germany.