FINAL REPORT TO

THE NATIONAL OPERATING COMMITTEE ON STANDARDS FOR ATHLETIC EQUIPMENT (NOCSAE)

THE USE OF SHOE ORTHOTICS TO REDUCE BONE STRAIN RATE

Introduction

The basis for undertaking this study was the prior work of Finestone et al published in Clinical Orthopaedics in 1999 entitled: A randomized clinical trial of the effect of custom biomechanical shoe orthotics on the incidence of stress fractures. In this study two types of custom biomechanical shoe orthotics worn in army boots were found to lower the incidence of stress fractures in Israeli army infantry recruits by 50 per cent. The mechanism by which these orthotics achieved this effect was not identified. The hypothesis of the current study was that training with custom biomechanical orthotics lessen the increases in bone strain and strain rates that are thought to occur with training fatigue. Since the etiology of stress fracture is cyclic loading of bone which produces high bone strain and strain rates any lowering of them should lower stress fracture risk.

We hypothesized that:

1. Strain rate at a stress fracture site in the distal tibia is decrased significantly with the use of either a semirigid polypropylene biomechanical shoe orthotic or a soft Pelite biomechanical shoe orthotic.

2. The magnitude of the fractional incrase in tibial strain rate that occurs when muscles become fatigued is decreased significantly by using either a semirigid polypropylene biomechanical shoe orthotic or a soft Pelite biomechanical shoe orthotic.

In the first part of the experiment in vivo tibial strain measurements were made in nine members of the research team during treadmill walking while wearing Nike Air shoes and army boots with and without soft and semirigid biomechanical orthotics. Measurements were also made during free walking before and after a two kilometer run done at the subject’s own pace, on an outdoor running track. Subjects wore either Nike Air running shoes with or without soft or semirigid biomechanical shoe orthotics for the run. The study population was diverse in age, sex and physical fitness, and consisted of both recreation athletes and people who did not do regular physical training. Strain measurements were made via 3 strain gauged bone staples inserted percutaneously in a 30 degree rosette pattern in the medial cortex at the level of the mid diaphysis. The major conclusions of this phase of the experiment were: 1) When worn within army boots semirigid biomechanical orthotics significantly lowered tension and shear strains and compression, tension and shear strain rates during treadmill walking. Soft biomechanical orthotics significantly lowered the compression and shear strains and strain rates. 2) When worn within Nike Air running shoes semirigid biomechanical orthotics lowered tension and compession strains, but the soft orthotics did not have any significant effect.3) No significant differences in the pre and post run strains and strain rates in five subjects wearing Nike Air running shoes and in three subjects wearing orthotics were found.

In the second part of the experiment a population of physically conditioned subjects was recruited. All were members of a special force anti-terrorist unit. The experimental protocol was altered as follows because of an observation in the first part of the experiment that army boots and Nike Air running shoes may interact with orthotics differently and because the commando recruits were reluctant to have three gauges inserted into their tibias: 1) A single axial gauge was inserted in an open surgical procedure, in the medial cortex of the mid-tibial diaphysis and a second gauge to be used as a backup in the event of gauge failure, was inserted in the distal tibial. 2) Subjects ran at a pace of 13 km/hr, for a series of two kilometer runs on a treadmill in Nike Air shoes, in Nike Air shoes with semirigid orthotics, and in Nike Air shoes with soft orthotics. Subjects also ran for one kilometer in army boots, in army boots with semirigid orthotics, and in army boots with soft orthotics.The order of the six runs was randomized for each subject. Tibial strain measurments were made during the runs, as well as before and after each run.

Materials and Methods (Part 2 of the study):

Nine members a special forces anti-terrorist unit mean age 32.4 (range 26-40 years), mean height 183.9 (range 178-189 cm), mean weight 82.8 (range 67-96 kg) volunteered to be subjects for in vivo tibial strain measurements. All subjects received explanations of the goals, risks and benefits of their participation in the experiment and signed informed consent. The experimental protocol was approved by the Research Ethical Committee of the Huddinge University Hospital where the study was conducted. All subjects were healthy and physically conditioned by their unit’s vigorous training regimen and had no prior history of medical problems.

Prior to the study each participant was prescribed semi-rigid and soft composite biomechanical orthotics fabricated from neutral subtalar casts. The semi-rigid orthotic

consisted of a ¾ length polypropylene module with an integrated neutral heel post and a full length covering of PPT (an open cell polyurethane foam), with a top cover of cambrel(Langer Biomechanics Group Inc., Deerfield New York). The thickness of the polypropylene module was determined by the recruit’s weight. The soft biomechanical orthotics (Eshed Advanced Orthopedics Ltd., Bene Brak, Israel) were full length orthotics with neutral hindfoot posts and were molded from three layers of polyurethane of different density (grade 80 upper layer, 60 middle layer and 80 lower layer). One month prior to the study the participants were also given Nike Air Max running shoes and Israeli Army infantry boots, with double layer soles (inner layer 45 durometer and outer layer 90 durometer polyurethane). The weight of the boots was 1600 gr. for shoe size 45. The study participants were instructed to “break-in” the shoes prior to the strain gage measurements. All orthotics and shoes were commercially purchased to avoid conflict of interest.

Each subject was asked to complete the same fatigue protocol, with only the order of the activities randomly varied between the subjects. Subjects performed serial two km treadmill runs at a rate of 13 km/hr while wearing Nike Air Max shoes without orthotics, with soft biomechanical orthotics and with semi-rigid biomechanical orthotics. Subjects performed one km treadmill runs while wearing army shoes without orthotics, with soft biomechanical orthotics and with semi-rigid orthotics. Tibial strains were recorded while walking on a treadmill at 5 km/hr immediately before and after each run while wearing the same shoe gear as was worn for the run. Tibial strain recordings were also recorded during the runs, five minutes after the beginning of each run. No rest was allowed between the runs other than the time necessary to complete the baseline treadmill walking measurements and for change of shoe gear.

In vivo strain measurements

Strain gauged staples made from 16 x 15 mm bone staples (3M Health Care, St Paul, Minnesota, USA), with two MicroMeasurements strain gauges (Measurements Group Inc, Raleigh, North Carolina, USA) types EA-06-031DE-350 and EA-06-031EC-350 mounted perpendicular to each other on the underside of the staple were used to measure in vivo axial tibial strains. Strain gauged staples were inserted in an open surgical procedure in the medial aspect of the mid and distal tibial diaphysis. The raw strain gauged staple signals were amplified by a strain gauge conditioner (Model 2120A, Measurement Group Inc, Raleigh, North Carolina, USA). Data were sampled at 1000 Hz and recorded with digital data collection software (Bioware, Kistler, Switzerland). The distal tibial strain gauged staple was used as a back up gauge in the event of a technical problem with the mid tibial gauge.

Surgical Protocol

Surgical implantation of the strain gauged staples was performed on an outpatient basis. Surgical implantation was performed in the morning, and the staples were removed the same day after completion of the experimental protocol.

The left leg was prepared and draped and surgical anesthesia achieved at the site of tibial strain gauged staple insertion with 2.5 ml 1% lidocaine and 2.5ml of 0.5% of buperocaine. First a 2.5 cm skin incision was made over the mid diaphyseal medial tibal surface. Dissection was carried down to expose the periosteum. A drill guide was used to drill two holes through the cortex to a depth of 4 mm. Using a specially designed insertion tool the strain gauged staple was inserted into the pre-drilled holes to a depth of 4 mm. Insertion of the distal tibial gauge was similiarly done. The surgical wounds were left opened and a gauze dressing placed loosely over each of the staples. At the end of experiment the strain gauged staples were removed and the skin wounds closed with two sutures. Prophylactic oral clindamycine were given at the time of staple insertion and removal.

Data Analysis

Data were processed using a custom written computer program running under Windows which takes the digitized amplified strain gauged staple outputs, filters them at 5 Hz, derives the peak to peak maximum axial tension and compression strains and the maximum tension and compression strain rates. The maximum strain rates were calculated by scanning 10 millisecond intervals along the tension and compression strain outputs.

Statistics were calculated using SAS. The statistical design was paired. The major outcome variables were the means of the peak to peak axial compression and tension strain and maximum compression and tension strain rates for treadmill walking before and after each of the runs and during each of the runs. Means were calculated for each of the activities on the basis of four consecutive strides. Repeated measures ANOVA (analysis of variance) followed by post hoc Duncan’s tests for significant differences were used to test whether the mean of each of the major outcomes differs between the type of shoes and orthotics worn. Differences were considered significant at p < 0.05.

Results

All of the subjects successfully completed the experimental protocol. There were no malfunctions of the mid tibial gauges, so their output was used for data analysis. Two of the distal gauges malfunctioned during the experiment. Peak to peak strains were calculated instead of separate compression and tension strains because of problem determining the baseline exactly in two subjects. While all subjects returned to their regular training within 10 days after the experiment, two had residual pain at the site of the distal strain gauged staple. This pain resolved after 12 weeks in one subject, but in the second was still symptomatic after 6 months. His blood count and sedimentation rate were normal. An MRI showed no evidence of osteomylitis, but soft tissue fibrosis under the surgical scar was present.

The peak to peak strains during treadmill walking while wearing army boots were significantly higher than while wearing Nike Air shoes (Fig. 1). When either soft or semi-rigid orthotics were worn together with either of the shoes the peak to peak strains were significantly lowered and there was no difference between the shoes.

Both compression and tension strain rates while wearing army boots were significantly higher during treadmill walking than while wearing Nike Air shoes (Fig. 2). Both orthotics significantly lowered the compression and tension strain rates when worn together with army boot, but only the compression rates when worn together with Nike Air shoes.

The peak to peak strains during treadmill running while wearing Army boots were significantly higher than while wearing Nike Air shoes (Fig. 3). When soft orthotics were worn together the Nike Air shoes peak to peak strains were significantly lowered. Wearing either of the orthotics with the army boots did not significantly affect the peak to peak strains.

Compression and tension strain rates while wearing army boots were significantly higher during treadmill running than while wearing Nike Air shoes (Fig. 4). Wearing either orthotic significantly increased the compression and tension strain rates when worn together with Nike Air shoes. When combined with the army boots only the tension strain rate significantly increased.

No statistically significant difference was found between the pre and post run treadmill walking tension and compression strains or strain rates (Fig. 5) with any combination of shoes and/or orthotics, suggesting that there is muscular fatigue in these experiments.

Conclusions

This study indicates that the use of custom biomechanical shoe orthotics (either soft or semi-rigid) with both running shoes and army boots is beneficial during training, which involves mostly walking, because it lowers walking strains and strain rates. The results of this study do not support the study hypothesis that custom biomechanical shoe orthotics decrease strain and/or strain rates during running, or following a fatiguing protocol. Indeed, strain rates increased during running when the orthotics wre used in conjunction with either the Nike shoes or the army boots. This suggests that under the training conditions of this study, the orthotics either have no effect, or could be detrimental. We cannot conclude that either orthotic will lower the incidence of stress fractures. Therefore, the use of such orthotics in subjects whose primary activity is running cannot be recommended for prevention for stress fractures.

Publications

1. Milgrom, C.; Finestone, A.; Levi, Y.; Simkin, A.; Ekenman, I.; Mendelson, S.; Millgram, M.; Nyska, M.; Benjuya, N.; and Burr, D.: Do high impact exercises produce higher tibial strains than running? Brit. J. Sports Med., 34:195-199, 2000.

2. Milgrom, C., Simkin, A., Eldad, A., Benjuya, N., Ekenman, I., Nyska, M. and Finestone, A.: Using bone’s adaptation ability to lower the incidence of stress fractures. Am. J. Sports Med., 28 245-51, 2000.

3. Milgrom, C.; Finestone, A.; Simkin, A.; Ekenman, I.; Mendelson, S.; Millgram, M., Nyska, M.; Larsson, E.; and Burr, D.: In vivo strain measurements to evaluate the tibial bone strengthening potential of exercises. J. Bone and Joint Surg., 82-B: 591-594, 2000.

Bars represent standard deviations

Bars represent standard deviation

Bar represents standard deviation

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