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Descriptions of the meaning of symbols used to BS EN ISO 15223-1:2016 - Medical devices - Symbols to be used with medical device labels, labelling labeling and information to be supplied - Part 1: General requirements is at Annex A.

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What is involved in a clinical gait analysis test? That is, how is it performed?

Since 1984, clinical gait analysis using Vicon Motion System products has involved the measurement of the patient's gait pattern with specialized technology. In the interim, technological improvements have enhanced the ease of use, reliability, sensitivity and specificity of the analysis, but the method has remained unchanged. The medical practitioners and health professionals at each clinic document the pertinent medical history and physical presentation, and this collection of manual and captured information provides the basis upon which treatment decisions are made.

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Initial presentation

Among the first examinations of the patient in the gait analysis laboratory is the careful observation by the clinician of the patient, barefoot and, perhaps, in orthoses, as she/he walks along a smooth, level pathway. Video recordings can be taken provide a qualitative documentation of how a person walks, affording an opportunity to evaluate the "smoothness" or "fluidity" of a gait pattern. The ability to obtain close-up views of a specific motion and the use of slow motion greatly enhance the observer's ability to evaluate the patient's walking pattern. For example, close-up views of the feet provide a means to evaluate hind foot position and motion.

Physical examination

The patient usually undergoes an extensive physical examination of their status at rest. The specific measurements depend somewhat on the pathology being evaluated. These measures may include the passive lower extremity joint motion, joint and muscular contracture, muscle strength and tone, bony deformity, and neurological assessment. This information may then be correlated with gait data to help determine the potential causes of the patient's gait deviations. However, the standard clinical examination used in isolation is limited in its capacity to offer diagnostic information because the effects of body position, gravity, and walking result in changes in functional demands that cannot be fully appreciated in a manual examination [4].

Kinematics, kinetics and electromyographic assessment

Central to a clinical gait analysis are the additional quantitative measurements of the patient's walking pattern provided from several different technologies.

Vicon motion capture systems (CLASS Im)

Passive retroreflective markers are placed on the surface of the patient's skin and aligned with specific bony landmarks and joint axes.

As the patient walks along a straight pathway in the laboratory, the locations of these markers are monitored with a three-dimensional motion data capture system comprising 8–12 optical cameras, all interfaced to a central controlling computer. Each of these cameras is equipped with an array of light-emitting diodes that strobe in synchrony with the camera shutters to illuminate the pathway with infrared or near infrared light. The light, which cannot be seen by the patient (and therefore does not distract the patient), is reflected by the markers back to the cameras. Computer programs allow the determination of the three-dimensional locations of the markers in space and time using photogrammetry and is analogous to the way depth is perceived in human vision with two eyes. Marker position data allow for the mathematical computation of the angular orientation of particular body segments as well as the angles between segments (ie, joint angles), collectively referred to as kinematics [2].

Ingestion of data from third-party ground reaction force plates

All Vicon motion systems support ingestion of data from approved third-party manufactured force plates to generate ground reaction force vectors. Multi-component force platforms imbedded in the walkway provide a measure of the net reactions between the foot and the ground as the patient walks along the pathway. These data may be assessed directly or used to calculate loads found in and across the joints of the lower extremity. These joint loads (referred to as kinetics [3]) are computed analytically from relationships drawn from physics that combine the simultaneously acquired kinematic information and estimates of limb mass and inertial properties [6].

Ingestion of data from third-party electromyography equipment

All Vicon motion systems support ingestion of data from approved third-party manufactured electromyography equipment (EMG). Electrodes placed on the surface of the skin or inserted as fine wires (smaller in diameter than human hair) into specific muscles allow muscle activity (expressed as action potentials) to be monitored as the patient walks along the laboratory pathway, through an approach referred to as dynamic electromyography. The EMG signal gives information concerning the "on-off" activity of a muscle. This information can be used with joint kinematic and kinetic results to better understand the patient's neuromuscular abnormalities [4, 7].

Gait laboratory staff competencies

A typical gait analysis test can take from one to two hours, depending on the particular evaluations performed and on the cooperation, behaviour, and gait complexity (ie, involvement) of the patient. Usually a clinical scientist, physiotherapist or kinesiologist works directly with the patient, and a more technically oriented person, such as an engineer or technician, manages the computer and measurement system operation during the test.

Institutional standards

It should be noted that not all clinical gait laboratories operate in this fashion. Some do not have the technology to provide three-dimensional body marker data and analytical processes that produce three-dimensional joint rotations (flexion-extension, abduction-adduction, and internal-external rotation) for interpretation. These clinical gait analysis laboratories use less sophisticated technology that collects and processes motion data based on the assumption that all of the rotations occur in the sagittal plane of the body. This assumes that all motions associated with gait can be appreciated from a side view of the patient. While this might not be unreasonable in the analysis of normal ambulatory motion (except perhaps for ankle/foot motion), it is clearly ill-advised for clinical decision-making in cases of pathological gait where three-dimensional motion is commonplace [11].

Moreover, at other facilities, a clinical gait analysis might be limited to a video recording and the measurement of certain gait stride and temporal parameters such as velocity, cadence, stride length, step length and percentage of stance/swing. While the video record is a useful tool in developing and substantiating visual impressions, it is inappropriate to "measure" joint and segment gait kinematics directly from the two-dimensional video imagery, even though some commercially available hardware and software products do this. With respect to stride and temporal parameters, these are "outcome" measures and provide an indication of the level of function when compared to normal values. They do not, however, give an indication of the cause of the gait abnormality and are, consequently, of limited value in clinical decision-making.

Similarly some laboratories capture only kinematic data so do not have access to the additional insights of kinetic information in their treatment decision making.

How are gait data interpreted?

Depending on the laboratory capabilities, the process of gait data collection, as described above, yields the following:

  • Video image recordings (Vicon Bonita, Bonita 2 and Vue cameras)
  • Clinical measures
  • Stride and temporal gait data, such as step length, cadence, and walking speed (Vicon motion capture systems)
  • Three-dimensional joint and segment motion plots (kinematics), (Vicon motion capture systems)
  • Three-dimensional joint torque or moment and power (kinetics) results (Vicon motion capture systems with third-party force plates)
  • Electromyographic (EMG) tracings (Vicon motion capture systems with third-party EMG)
  • A measurement of metabolic energy expenditure (independent devices used by laboratory and out of scope)

These parameters are then evaluated to identify abnormalities, using a database of normal, typically developed subjects and knowledge of normal gait biomechanics as baseline. Deviations from normal are always interpreted in the context of their relative impact on gait function. These multiple sources of data provide useful redundancy, allowing corroborating information to be identified and conflicting observations to be understood.

A challenge in gait data interpretation is to identify the primary problems that perhaps need to be addressed and then to recognize secondary abnormalities, produced as result of the primary problems, and to appreciate compensatory mechanisms (ie, strategies the patient uses to overcome the impairment). For example, the primary problem of a crouched knee (as a result of the hamstring tightness and hip extensor weakness) can produce secondary "abnormalities" at the hip (reduced hip extension in stance) and pelvis (the posterior position), all of which should be resolved with hamstring intervention.

The role of the interdisciplinary team

At major clinical laboratories, gait data are interpreted by a team that consists of the orthopedic surgeon to whom the patient was referred and the clinical scientist, physiotherapist and/or kinesiologist who collected data. At times, the engineer or technician, who assisted in the data collection, or the biomechanical engineer who developed the mathematical models used to process data, is involved, if questions of data quality arise or if some previously unseen walking mechanism is encountered. Most laboratories will use multi-centrecenter, internationally validated conventional gait models for routine clinical analyses [12]. In general, it is important that the team has at least a rudimentary understanding of the gait model used to produce the results, in addition to a well-developed understanding of normal gait. This knowledge base, underpinned by experience gained from the examination of many pre- and post-treatment cases, is essential to produce a proper interpretation and treatment decision. The use of validated models facilitates the exchange of experience through multi-centre center collaborations and exchange using a common methodology at academic conferences.

What additional information is provided through gait analysis that augments observational analysis?

For gait analysis to be a useful tool in clinical decision-making, it must provide information that is not available through more traditional methods of evaluation. Use of clinical gait analysis systems augments visual observations by using:

  • A quantitative description of complex movements that are not only multi-planar, but which also involve multiple lower extremity joints and the upper body
  • An indication of the associated muscle activity
  • A consideration of joint kinetic patterns
  • An opportunity to learn from documented treatment outcomes

With this additional information, the clinician can be more confident about identifying gait deviations, determining their potential causes, and appreciating the treatment outcome. This entire process will ultimately lead to new treatment approaches and a reduction in the use of less effective interventions. The following examples are intended to illustrate how gait analysis can benefit the clinician in treatment decision-making.

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I. Quantitative description of motion

Identifying abnormal motion in the transverse plane

Rotational abnormalities are a common problem in many neuromuscular disorders, such as cerebral palsy and myelomeningocele. Torsional bony deformities, identified during the clinical examination, may be present in the femur and/or tibia. Abnormal segment positions may be seen in gait, such as asymmetrical pelvic rotation with one side of the pelvis retracted or externally rotated and the contralateral pelvis protracted or internally rotated. Finally, there may be abnormal rotations at the hip, knee and ankle joints during gait as well.

During observational gait analysis, the focus for identifying rotational abnormalities is usually on foot progression (the orientation of the foot to the direction of progression) and the position of the knees. That is, if the knees are pointing inwards, it is assumed that there is internal hip rotation and/or femoral anteversion. Gait analysis can document segment and joint motion in the transverse plane to allow accurate identification of the location of rotational abnormalities, including pelvic motion, which can be difficult to determine visually.

A treatment approach based on a reasonably symmetrical clinical assessment of the rotational abnormalities and visual impression of symmetry in gait may result in an unexpected treatment outcome. This may help explain some of the unpredictability in surgical outcomes in the patient with cerebral palsy in which surgical decisions were made without a pre-operative gait analysis being performed. One of the most common problems observed in a patient with cerebral palsy is a drop foot in late swing. In some cases, the foot or toe may contact the ground in swing, resulting in reduced stability and possible falling. Drop foot creates a less stable position of the foot on landing, referred to as a toe initial contact. The primary cause of drop foot is generally thought to be excessive equinus in swing due to tibialis anterior muscle weakness, and/or ankle plantar flexor tightness or spasticity. In those patients, where normal or at least a neutral passive range of motion is possible, an ankle-foot orthosis is often prescribed.

During observational gait analysis, when a drop foot is suspected, all too often, the observer concentrates on the orientation of the foot in late swing and at initial contact, (the foot is pointed downward in swing resulting in a toe contact). A common error in these circumstances is to presume that this orientation is due to ankle position alone, that a plantar-flexed ankle only causes the foot to point downward. It is important in cases such as these to appreciate not only the role of the ankle, but also that of the knee in positioning the foot segment, which is readily identified when using quantitative clinical gait analysis.

II. Indications of the associated muscle activity

Although dynamic EMG has its limitations (ie, the amplitude information is limited unless directly related to a known force [15]), this technique is the only way to determine whether a particular muscle is active during gait [16]. One can usually predict that a group of muscles is active, such as the knee extensors in a patient in crouch. However, the entire muscle group may not be active. In the majority of patients with cerebral palsy, the rectus femoris is active in mid swing and during the Duncan Ely test, but the vastus medialis and lateralis are not [13]. Similarly, to determine the cause of hind foot varus, all the potential contributors need to be assessed [4]. An examination of EMG data in conjunction with the joint kinetics can also provide more information about the cause of internal joint moments.

Determining posterior tibialis activity during gait

One common problem in cerebral palsy of the spastic hemiplegia type is varus deformity of the hind foot. Intramuscular (fine-wire) EMG helps determine the possible role of the tibialis posterior muscle in producing this deformity.

Without understanding the potential causes of the deforming forces and associated abnormal motion, treatment decision-making can be, at best, an educated guess. Electromyographic data can provide information about possible contributors to the abnormal motion. These data, in combination with joint motion and kinetics, can help determine whether deforming forces are associated with the abnormal EMG.

III. Indications of the associated joint kinetics

One of the more common problems experienced in ambulatory patients with myelomeningocele at the L4 or L5 functional level, is a knee valgus thrust during the initial weight-bearing phase of the gait cycle [17]. It is believed that this motion must compromise the medial soft tissue structures of the knee over time. The most common treatment for this problem is the knee-ankle-foot orthosis (KAFO) which is meant to protect the medial knee [18].

The body's response to the valgus thrust at the knee is a net knee adductor moment. Joint kinetic data, specifically the net knee coronal plane moment in-stance, would substantiate the presence of a knee valgus thrust; that is, the net internal knee moment would be an adductor moment.

IV. An opportunity to learn from documented treatment outcome

In many laboratories, a routine part of clinical gait analysis is to re-evaluate each preoperative patient about a year after surgery. At this time, the preoperative test procedures are repeated, so that comparisons can be made. The clinician becomes more aware of the specific outcomes related to the patient and can begin to understand the complex relationships between primary, secondary and compensatory gait abnormalities. The wealth of knowledge that has accumulated over time using this systematic approach for the treatment of gait abnormalities in cerebral palsy is phenomenal (as evidenced by the body of literature cited).

This systematic approach also has other benefits. The quantitative nature of gait analysis facilitates prospective clinical research, and the development of a large database of pre- and postoperative analyses facilitates retrospective research. Routinely doing postoperative gait analyses enables the systematic evaluation of the effects of surgery on specific populations, or the outcomes of specific procedures. This has led to many beneficial changes in the approach to surgical treatment of patients with cerebral palsy. The approach allows an increased understanding of the ramifications of certain procedures, such as gastrocnemius lengthening [19] and the surgical treatment of equinovarus foot deformities [20]. Gait analysis techniques have resulted in the development of the rectus femoris transfer [21, 22, 23], an understanding of the difference in effectiveness of the rectus femoris transfer as compared with the rectus femoris release [24], and an appreciation of the significance of the location of distal rectus transfer site [25]. It has also led clinicians away from specific procedures, such as the hip adductor transfer [26]. Another benefit of routinely implementing gait analysis is that it provides a means to directly assess the effects of similar procedures, such as the medial hamstring versus medial and lateral hamstring lengthening [27] and hamstring lengthening with [22] or without a simultaneous rectus femoris procedure [28], on functional outcome.

Which patients can benefit from a clinical gait analysis?

Quantitative clinical gait analysis techniques are appropriate for any adult or child who has a gait problem that requires treatment. Because of the complexity of gait abnormalities in neuromuscular disorders, gait analysis is most commonly performed in this patient population. Gait analysis is appropriate for guiding decision-making on management in such disorders as cerebral palsy, stroke, traumatic brain injury and myelomeningocele, among others. Because of the complexity and expense of the test, gait analysis is primarily used as part of the surgical decision-making process when all conservative treatments have been exhausted and surgical intervention is being considered. However, gait analysis is not limited to this application only. Questions concerning bracing issues and medication efficacy can be addressed using gait analysis techniques. For example, is the brace performance or drug intervention (ie, Botulinum toxin, baclofen) consistent with the prescriptive objectives? Evaluation of the rate of deterioration in progressive disorders that affect gait can also aid in understanding a patient's abilities and directing countermeasures. As described above, another valuable function of gait analysis is assessing the efficacy of surgical intervention. Routine analyses of postoperative functional status provides the clinician with more objective information to evaluate the effects of treatment as well as a basis for determining the next steps in the treatment plan.

A number of factors must be considered when referring a patient for gait analysis. At many centrescenters, the patient must be ambulatory, with or without assistive devices, for at least 10 consecutive steps. The patient must be able to follow simple directions and to behaviourally tolerate the placement of reflective markers and EMG electrodes on the skin. The level of patient cooperation influences testability, given the time required for a typical gait analysis, particularly in cases of severe cognitive impairment. If a patient has orthoses, testing with and without the devices may be required to address clinical questions concerning brace wear. Usually, testing is conducted with the patient using any necessary walking aids. A full gait analysis that includes all the above parameters takes approximately one to two hours.

Perhaps the most important consideration in using clinical gait analysis is the proper formulation of the specific questions to be addressed by the analysis. For example, what is the cause of the tripping/falling and what is the etiology of idiopathic joint pain? Such questions need to be asked to properly direct the application of the technology and the associated interpretation process. Certainly there is a temptation to believe that the technology, specifically, the computer, not only aids in the analysis, but can also direct the analysis. As illustrated throughout this article, the experience and knowledge of the professionals who collect and interpret gait data are essential to clinical gait analysis.

As previously mentioned, a referral for gait analysis is usually made when all methods of conservative treatment have been tried and surgical options are being considered. This typically occurs after the patient has reached an ambulatory plateau and/or when orthopedic concerns necessitate treatment (ie, hip subluxation or severe joint contractures). In patients with cerebral palsy, multi-level surgeries are now performed to address all dysfunction during one surgical intervention. This not only reduces a patient's exposure to anesthesia, but it also reduces the need for frequent hospitalizations and periods of rehabilitation. Gait analysis is invaluable in identifying the multiple areas of impairment that are difficult to understand by observation and clinical assessment alone. For example, when used as a preoperative tool, the child with cerebral palsy may often need only one surgical package of treatment during the growing years.

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  • Repeatability: The variation observed when the same system measures the same location or parameter repeatedly.
  • Reproducibility: The variation observed when different systems measure the same location or parameter repeatedly.

Soft tissue

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artifact

The engineering and physics problem of capturing and tracking 3D points in space has been solved and well understood within Vicon motion capture systems. However, clinical concerns remain over the loose association between the measurements of points on the skin surface and the underlying bone, commonly described as the soft tissue artefact [47]. Challenges also remain in reliably capturing bony landmark axial rotation, in particular, of the thigh [48].

Due to inaccuracies related to working with biological systems [49], there are limitations in the way 3D motion data are acquired. Markers attached to the skin move with respect to the underlying bones that they are intended to represent. Soft tissue artefact artifact (STA) arises from movement or deformation of the subcutaneous tissues associated with muscular contractions, skin movement and inertial effects. The extent of STA for any movement depends upon the physical characteristics of individuals, marker locations and the nature of the movement task performed.

Many researchers and gait laboratories have proposed techniques to move away from existing predictive approaches by instead discovering joint centres centers and axes functionally, and fitting the data to an idealized joint model that also incorporates some form of soft tissue artefact artifact compensation. This work continues and, like the earlier innovations, will gain wide acceptance only after it has been validated through close collaborative efforts between clinicians, scientists and engineers in laboratories, academia and industry.

To ensure where possible that spatio-temporal marker measurement artefact artifact is not introduced into the wider problem, Vicon motion capture systems are designed to achieve performance in all variants to always be below the anticipated soft tissue artefactartifact. The performance upper limit is defined within the published Declaration of Conformity and is common for all published CLASS Im product offerings.

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Peters et al [49] undertook a systematic review to critically evaluate the quantification of STA in lower limb human motion analysis. It has a specific focus on assessing the quality of previous studies and comparing quantitative results. A specific search strategy identified 20 published articles or abstracts that fulfilled the selection criteria. The quality of the articles was evaluated using a customized critical appraisal tool. Data extraction tools were used to identify key aspects reported in the articles. Most studies had small sample sizes of mostly young, slim participants. Eleven of the reviewed articles used physically invasive techniques to assess STA. STA was found to reach magnitudes of greater than 30 mm on the thigh segment, and up to 15 mm on the tibia. The range of soft tissue artefact artifact reached greater than 25 mm in some cases when comparing the results of reviewed studies.

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Using a minumum of four optical motion cameras, resolution of the distance between the centres centers of two static 14 mm spherical markers located within a volume not less than 4 m x 4 m x 1.5 m to within 1 mm mean; 1 mm Standard Deviation; sample size no less than 1,000.

This equates to one order of magnitude (1 mm3) better than reported skin movement artefact artifact 10 mm3 [47].

Analogue digital conversion

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Outputs from 3rd Party Kinectic and Electromyographic devices is ± 5 V  to ± 10 V. Measurement artefact artifact is assessed at 0.2–0.1% error.

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