Anton Arndt

Effects of fatigue and load variation on metatarsal deformation measured in vivo during barefoot walking

This in vivo study presents information to assist in the understanding of metatarsal stress fracture etiology. The aims were (a) to provide a fundamental description of loading patterns of the second metatarsal (MTII) during barefoot walking, and (b) to investigate the hypothesis that MTII dorsal strain increases with fatigue and external carrying load. Dorsal MTII strain was measured in vivo under local anaesthetic with an instrumented staple in eight subjects. Experimental conditions were external loading with a 20 kg backpack and pre- and post-fatigue. M. flexor digitorum longus electromyography tentatively indicated fatigue after an extended walking treatment. A reproducible, cyclic temporal pattern of dorsal MTII surface deformation was described. Mean peak compression and tension strains in unloaded barefoot walking were -1534 +/- 636 and 363 +/- 359 muepsilon, respectively. Mean peak compression strain rate (SR) was -4165 +/- 1233 muepsilon/s. Compression strain increased significantly (alpha=0.05) both with the addition of the backpack and post-fatigue while maximum tension decreased significantly post-fatigue. SR increased significantly with the addition of the backpack. The highest plantar force time integrals were recorded underneath the heads of metatarsals II-V for all conditions (1561Ns pre-fatigue, without backpack; 2123Ns post, with). EMG and plantar pressure data presented a comprehensive description of biomechanical parameters influencing dorsal MTII deformation and alterations in strain following two experimental conditions were suggested as contributing factors in the pathogenesis of metatarsal stress fractures.

Differences in Ankle-Joint Complex Motion During the Stance Phase of Walking as Measured by Superficial and Bone-Anchored Markers

Bones of the ankle-joint complex are difficult to represent with superficial markers and an invasive approach is required to quantify skin movement artifact. Three-dimensional coordinates during gait were first calculated from sets of three superficial markers located on both the lower leg and heel. Intracortical pins equipped with external marker arrays were subsequently inserted in the tibia, talus and calcaneus for further trials. Tibiocalcaneal and talocalcaneal joint helical axis component rotations were calculated. Intersubject motion patterns were comparable. Root mean square (RMS) differences were chosen to describe discrepancies between temporal intracortical pin and superficial marker curves. Results for the tibiocalcaneal rotations inversion / eversion, plantarflexion / dorsiflexion and abduction / adduction were 2.5°, 1.7° and 2.8° respectively. Inversion / eversion about the talocalcaneal joint showed an RMS difference of 2.1°. A systematic over- or underestimation of superficial relative to bone-anchored markers could not be determined.

Stability of the Birmingham hip resurfacing arthroplasty at two years A RADIOSTEREOPHOTOGRAMMETRIC ANALYSIS STUDY

The Birmingham hip resurfacing metal-on-metal arthroplasty was introduced in 1997 and has shown promising short- to mid-term results. We used radiostereophotogrammetric analysis (RSA) to study the stability of 20 resurfacing arthroplasties over a follow-up period of 24 months. Radiological examinations for RSA were performed immediately after surgery and at two, six, 12 and 24 months after operation. Precision and detection of migration thresholds (non-zero movement) were calculated. All the results corresponded well to those found in similar experimental arrangements with standard hip prostheses. Migration of the cup and vertical and mediolateral migration of the head were calculated. The values were low at two years compared with those of earlier studies of cemented femoral components in conventional total hip replacements indicating that there was no evidence of excessive early migration or loosening of the components.

Functional units of the human foot

Functional units in the human foot provide a meaningful basis for subdivisions of the entire foot during gait analysis as well as justified simplifications of foot models. The present study aimed to identify such functional units during walking and slow running. An invasive method based upon reflective marker arrays mounted on intracortical pins was used to register motion of seven foot bones. Six healthy subjects were assessed during walking and four of them during slow running. Angle-angle diagrams of corresponding planar bone rotations were plotted against each other and used to establish functional units. Individual functional units were accepted when the joints rotated temporally in phase and either (i) in the same direction, (ii) in the opposite direction, or (iii) when one of the two joints showed no rotation. A functional unit was generalized if all available angle-angle diagrams showed a consistent pattern. A medial array from the navicular to the first metatarsal was found to perform as a functional unit with parts rotating in the same direction and larger rotations occurring proximally. A rigid functional unit comprised the navicular and cuboid. No other functional units were identified. It was concluded that the talus, navicular, and medial cuneiform should neither be regarded as one rigid unit nor as one segment during gait analysis. The first and fifth metatarsals should also be considered separately. It was further concluded that a marker setup for gait analysis should consist of the following four segments: calcaneus, navicular-cuboid, medial cuneiform-first metatarsal, fifth metatarsal.

Variation in the position and orientation of the elbow flexion axis

We analysed the axis of movement in the normal elbow during flexion in vivo using radiostereometric analysis (RSA). The results show an intraindividual variation in the inclination of the axis ranging from 2.1 degrees to 14.3 degrees in the frontal and from 1.6 degrees to 9.8 degrees in the horizontal plane analysed at 30 degrees increments. The inclination of the mean axis of rotation varied within a range of 12.7 degrees in the frontal and 4.6 degrees in the horizontal plane. In both planes, the mean axes were located close to a line joining the centres of the trochlea and capitellum. The intra- and interindividual variations of the axes of flexion of the elbow were greater than previously reported. These factors should be considered in the development of elbow prostheses.

A comparison of external plantar loading and in vivo local metatarsal deformation wearing two different military boots

The introduction of the M90 boot with a more flexible outer sole to military recruits in Sweden was accompanied by an increase in second metatarsal stress fractures. This study compared the new boot with the previous, stiffer model. A combination of external plantar pressure measurement (two subjects) and an in vivo measurement of dorsal metatarsal strain (six subjects) using strain gauge instrumented staples was implemented. Walking in both boot models resulted in increased plantar pressure under the heads of the lesser metatarsals and generally decreased loading under the remainder of the foot. Dorsal metatarsal tension increased for subjects wearing the new boot throughout a walking protocol.

Correction for sensor creep in the evaluation of long-term plantar pressure data

Plantar pressure measurements are frequently applied in the biomechanical assessment of lower extremity injury risk. Different conditions such as footwear, orthopedic insoles or movement tasks are usually compared by separate trials of the individual conditions. However, injury risk may be related to fatigue from long-term exercise and no actual measurements of plantar pressure during fatigue treatment have been published. A simple method is presented for determining sensor creep over a 3 h walking trial carrying a heavy load (49% of bodyweight). Plantar pressure measurements were conducted using Pedar insoles with capacitative sensors. Repeated standing trials were conducted and the total force underneath both feet measured under the assumption that this value should remain constant over time. The percentage fluctuation from the first such static measurement can be used to correct measured parameters of interest for the influence of sensor creep. The pressure sensor values increased by up to 17% after 3 h and the method presented permitted the correction of measured plantar pressure parameters to account for this sensor creep behaviour. Such correction appears necessary for correct interpretation of the fatigue effects on plantar loading. Creep correction as described here should be performed individually and separately in each long-term trial.

Force in the Achilles Tendon During Walking With Ankle Foot Orthosis

Background Ankle foot orthoses are used for postoperative treatment of Achilles tendon ruptures and decrease calf muscle electromyography activity during walking. Hypothesis Achilles tendon load decreases with increased restriction of dorsiflexion and is associated with decreased triceps surae activity. Study Design Controlled laboratory study. Methods In 8 subjects, the maximum force and rate of force development in the Achilles tendon were measured with an optic fiber technique, and the activity of the gastrocnemius, soleus, and tibialis anterior muscles was recorded using electromyography. Trial conditions were walking barefoot and wearing an ankle-foot orthoses set in 3 different positions: (1) locked at 20° of plantar flexion and with free plantar flexion but restricted dorsiflexion to (2) 10° plantar flexion and (3) 10° dorsiflexion, respectively. The design of the ankle foot orthoses did not provide heel support when fixed in a plantarflexed position. Results Maximum Achilles tendon force was highest at the ankle-foot orthoses setting of 20° plantar flexion (3.1 times body weight) and decreased to 2.1 times body weight during barefoot walking (P < .01). The rate of Achilles tendon force showed an increasing trend with less-restricted dorsiflexion. Soleus activity was 52% of mean barefoot walking activity at 3 20° plantar flexion (P < .001) and then increased as dorsiflexion was less restricted. Conclusion Weightbearing in ankle-foot orthoses when dorsiflexion is restricted beyond neutral may result in increased forces in the Achilles tendon compared with barefoot walking, despite reduced electromyography activity in the triceps surae and decreased rate of force development. Clinical Relevance If patients bear full weight in an ankle-foot orthoses locked at 20° plantar flexion without heel support, the maximum force in the tendon may exceed that encountered during barefoot walking.

A five-year radiostereometric follow-up of the Birmingham Hip Resurfacing arthroplasty.

The early designs of hip resurfacing implants suffered high rates of early failure, making it impossible to obtain valuable mid-term radiostereophotogrammetric (RSA) results. The metal-on-metal Birmingham Hip Resurfacing arthroplasty has shown promising mid-term results and we present here the first mid-term RSA analysis of a hip resurfacing implant. The analysis was performed in 19 hips at five years post-operatively. The mean acetabular component translation and rotation, and femoral component translation were compared with the previous RSA measurements at two and six months, and one and two years. There was no statistical significance (t-test, p < or = 0.05) between these consecutive movements, indicating the mid-term stability of the implant.

Modelling human musculoskeletal functional movements using ultrasound imaging

BackgroundA widespread and fundamental assumption in the health sciences is that muscle functions are related to a wide variety of conditions, for example pain, ischemic and neurological disorder, exercise and injury. It is therefore highly desirable to study musculoskeletal contributions in clinical applications such as the treatment of muscle injuries, post-surgery evaluations, monitoring of progressive degeneration in neuromuscular disorders, and so on.The spatial image resolution in ultrasound systems has improved tremendously in the last few years and nowadays provides detailed information about tissue characteristics. It is now possible to study skeletal muscles in real-time during activity.MethodsThe ultrasound images are transformed to be congruent and are effectively compressed and stacked in order to be analysed with multivariate techniques. The method is applied to a relevant clinical orthopaedic research field, namely to describe the dynamics in the Achilles tendon and the calf during real-time movements.ResultsThis study introduces a novel method to medical applications that can be used to examine ultrasound image sequences and to detect, visualise and quantify skeletal muscle dynamics and functions.ConclusionsThis new objective method is a powerful tool to use when visualising tissue activity and dynamics of musculoskeletal ultrasound registrations.