The Function of the Reciprocal Link in Paraplegic Orthotic Gait

Abstract

Hip-knee-ankle-foot orthoses (HKAFOs) are routinely used to allow people with thoracic level paraplegia to walk. However, many users give up the orthoses because of fatigue or inconvenience of use. Therefore, much research is focused on the design of orthoses that are easier to use and that provide a more energy-efficient gait. One of the main differences between HKAFO designs for these users is the design of hip joints, with one major group being those with reciprocally linked hip joints. If improvements are to be made to these features, the specific benefits and problems encountered in their use must be understood. The purpose of this review is to highlight the qualitative and quantitative evidence for mechanical function of the reciprocal link in HKAFOs used for walking by individuals with thoracic level paraplegia.

A review of the function and importance of a mechanical link between the hip joints of an HKAFO in qualitative terms highlights the fact that text on the function of the linkage is often sketchy and at times contradictory. At the same time, the importance placed on the function of the reciprocal link during the swing phase of gait is almost uniformly great. Seven published papers have assessed the function of the reciprocal linkage in a quantitative manner. The information reported in all these papers questions the importance placed on the reciprocal action during swing phase. However, support is provided for the underemphasized function of preventing bilateral hip collapse during the double support phases of gait.

Hip-knee-ankle-foot orthoses (HKAFOs) are routinely used for walking by people with thoracic level paraplegia. There are many different designs available, but a popular subset of these orthoses has hip joints that are linked to constrain sagittal hip motion to a reciprocal action. The hip joints are linked by a number of different mechanisms, but all are intended to provide similar function. The hip joints are restricted to a reciprocal motion in the sagittal plane, so that flexion of one hip joint is coupled to extension of the other. This linkage also restricts bilateral hip motion when standing. The literature reviewed refers to three orthoses with different types of reciprocal link: 1) the Louisiana State University Reciprocating Gait Orthosis (LSU-RGO); 2) the Advanced Reciprocating Gait Orthosis (ARGO); and 3) the Isocentric Reciprocating Gait Orthosis (IRGO). In general, this review treats the reciprocal link as a generic unit and does not distinguish between specific orthoses.

The LSU-RGO ¹ was developed in the early 1980s from the previous geared designs of the Ontario Crippled Children’s Centre, Toronto, Canada. This orthosis has two Bowden-type cables linking the hip joints. The outer casing of each cable is attached to the trunk section of the orthosis. The ends of one of the inner cables are attached posterior to the hip joint center of each hip (the back or posterior cable), and the ends of the other inner cable are attached anterior to the hip joint center of the hips (the front or anterior cable). Each cable can transmit force only when in tension; when in compression, the cables buckle. The ARGO ² is a single-cable variant of the LSU-RGO that was developed in the early 1990s by Hugh Steeper Ltd. (London, England). The cable is a linear bearing and can work in tension and compression. The actions of the LSU-RGO front cable in tension are undertaken by the ARGO cable in compression. The IRGO, ³ which was also designed in the early 1990s, is an improvement of the LSU-RGO that was developed at the Center for Orthotics Design in Campbell, California. A solid rocker bar attached behind the hip joint centers replaces the cable linkage and performs the function of the single cable of the ARGO.

The first part of this review deals with qualitative descriptions of the functions of the reciprocal link. Functions that are ascribed to the reciprocal link, the timing of these functions within the gait cycle, and the importance attached to these functions are explored. The second part of the review assesses the quantitative data available on the function of the reciprocal link.

QUALITATIVE REVIEW OF THE FUNCTION OF THE RECIPROCAL LINK

The most basic function of the linkage is to constrain the hips to a reciprocal motion between flexion and extension. ^2–27 Many articles state that hip flexion is produced by contralateral hip extension, ^{4,5,10,11,14–16,22,23,25,28} although two articles assert that stance hip extension is caused by swing hip flexion. ^3,19 This difference in the perceived use of the reciprocal link could well derive from the specific user population that was being targeted by the authors. If users had some hip flexion, but no hip extension, then it would be desirable for swing leg hip flexion to drive stance leg hip extension. If users had no ability to flex the hips, the opposite would be the desired use of the reciprocal link. Fewer articles acknowledge that the reciprocal link also has the effect of stabilizing the hip joints during weight bearing. ^{1,4,9,12,15,18–20,21,24–27} The tendency of the authors was to highlight either that the reciprocal link prevented bilateral hip flexion ^{1,12,19,20,21,25} or that the reciprocal link prevented bilateral hip extension, ^9,15 with only three articles mentioning both aspects of the function. ^4,18,27 Few articles describe the use of the reciprocal link both to provide a reciprocal action and to prevent bilateral hip motion. ^{1,4,12,15,18–21}

Along with the question of the functions of the reciprocal link, their timing within the gait cycle is of importance. Four articles note that prevention of bilateral hip motion occurs when both feet are on the ground (double support). ^15,23,25,27 It is clearly stated in four articles, ^4,18,19 and implied in six others ^{5,10,15,16,28,29} that the reciprocal action of the linkage occurs during the swing phase of gait. Of those articles that report both basic functions of the reciprocal link, four also distinguish between their actions at different points in the gait cycle. ^4,15,18,19

When dealing specifically with the reciprocal link of a dual-cable system, only four articles differentiate between the actions of the individual cables. ^4,15,18,30 Within these, there is a consensus regarding the function of the cables during the double support phases of gait. Beckman, ⁴ Scrutton, ¹⁸ and Dall et al. ³⁰ state that the posterior cable prevents bilateral hip flexion and the anterior cable prevents bilateral hip extension. Ogilvie et al. ¹⁵ state that the anterior cable prevents bilateral hip extension, but do not suggest what prevents bilateral hip flexion. There is no consensus on the specific function of the cables during swing phase. Beckman ⁴ and Scrutton ¹⁸ do not specifically relate reciprocal action to a particular cable. Ogilvie et al. ¹⁵ imply that the posterior cable provides the entire reciprocal action for hip flexion. Dall et al. ³⁰ state that, during swing phase, driving one leg into flexion by contralateral hip extension causes tension in the anterior cable and that pushing one leg into extension by contralateral hip flexion causes tension in the posterior cable. With such minimal data on the subject, it is impossible to reach a consensus. However, these points are of vital interest when it comes to interpreting quantitative results of direct measurement of cable functions.

Full information on the function and timing of the reciprocal linkage is not available in any individual article. Thus, complete information has to be drawn from a variety of sources, and this has led to contradictory evidence on the more specific functions of the reciprocal linkage. It seems that the mechanical function of a system that has been in use for so long cannot be definitely determined from the qualitative descriptions available in the literature.

Three articles emphasize the prevention of bilateral hip motion for standing stability. ^12,19,27 The importance attributed to the hip linkage mechanism when walking reciprocally ranges from being the sole driving force of hip flexion, ¹⁰ or a key or significant feature, ^1,16 to hip flexion being caused ^18,23,25,28 or assisted ^5,9,14 by hip extension during swing phase. One article suggests that the linkage could be a disadvantage rather than an advantage, restricting the walking performance of users. ³¹

QUANTITATIVE REVIEW OF THE FUNCTION OF THE RECIPROCAL LINK

The majority of articles describing the functions of the reciprocal link do not reference quantitative work that substantiates these descriptions. One article mentions that the effect of the reciprocal link is unproven and, therefore, only a theoretical advantage. ²⁷ In the second part of this review, quantitative evidence for the function of the reciprocal link is assessed, both by direct measurement and by indirect methods such as the measurement of energy expenditure and modelling.

Quantitative results from direct assessment of the function of the reciprocal link have been reported in three articles. Scrutton ¹⁸ measured the cable load of a dual-cable system; “working load” was found to vary during gait from 160 N to 220 N (35–48 lbf). No information was presented regarding cable differentiation. Petrofsky and Smith ³² measured the forces in both cables of an LSU-RGO using load cells while four subjects walked on different grades of slope. Although force was measured in both cables, results were presented as a graph with a single trace for each slope and it was unclear whether this line represented some form of averaged cable load or was the load from only one cable. Maximum force measured in the cables of an LSU-RGO when walking on level ground was 230 N (50 lbf). The distribution of the cable force with respect to the phase of gait cycle was also shown graphically. Although the beginning and end of the phases are difficult to determine, it was demonstrated that the tension in the link varied during the gait cycle. Dall et al. ³⁰ reported hip movement derived from measured cable force for six spinal cord injured patients walking in the LSU-RGO. It was found that no force was transmitted in the front cable during the swing phase for four of the six users, and the reciprocal function of the cable was not used to aid swing hip flexion. The other two users showed a peak cable tension of up to 5 Nm during some swing phases, and it was considered unlikely that this low value of maximum hip movement was the only driving force of swing phase. For one of these users, tension built up in the front cable during double support, so that it was bilateral hip extension during stance phase and not stance leg hip extension during swing phase that caused the tension in the cable. Therefore, the common assertion that stance leg hip extension causes contralateral hip flexion during swing phase was only true of one of six users. In contrast, the back cable was in tension for 97% to 100% of stance phase for all subjects, with maximum hip movement of 12 to 35 Nm. Thus, it was demonstrated that the function of preventing bilateral hip flexion was important for all subjects when walking in the LSU-RGO. Tension of up to 18 Nm was also recorded in the back cable during the later part of swing phase. It is possible that this finding demonstrates a restriction of hip flexion during later swing phase.

Several investigators ^27,33 have tried to assess the function of the reciprocal link indirectly by comparing the energy expenditure of patients walking in orthoses with and without reciprocally linked hip joints. The broad variation of population within such studies means that there can be a difficulty in isolating the functions of the reciprocal linkage from other differences between orthoses. Ijzerman et al. ²⁷ improved the isolation of cable function by comparing energy expenditure of subjects in an ARGO and in the same ARGO with the cable disconnected.

Hirokawa et al. ³³ measured the energy cost of the LSU-RGO over a range of speeds (0.1–0.4 m/s) and compared this to values for walking in the ParaWalker (an HKAFO with unlinked hip joints) obtained from the literature. At speeds slower than preferred walking speed (0.2 m/s), users had a lower energy cost walking in the LSU-RGO than in the ParaWalker. At preferred walking speed and faster, users had a higher energy cost walking in the LSU-RGO compared with walking in the ParaWalker. Ijzerman et al. ²⁷ measured oxygen cost and speed of patients walking in the ARGO and the same ARGO with the reciprocal link disconnected. The oxygen cost of patients with high thoracic lesions (slower gait) walking in the ARGO was lower than that of patients in the orthosis with the reciprocal link disconnected. The oxygen cost of patients with lower level thoracic lesions (faster gait) walking in the ARGO was higher than that of patients walking in the orthosis with the reciprocal link disconnected.

As speed of walking increases, the double support phases of gait decrease proportionally compared with swing phase. Hirokawa et al. ³³ and Ijzerman et al. ²⁷ postulate that, at slower gait speeds, the benefit of a reciprocal link supporting the hips during double support contributes to the lower energy expenditure of users walking in the orthoses with reciprocally linked hip joints. Conversely, at faster gait speeds, a restriction to ballistic swing by the reciprocal link during swing phase may contribute to the higher energy expenditure of users walking in orthoses with a reciprocal link.

Tashman et al. ²⁰ modelled the swing phase of ARGO gait. To create the hip extension necessary to gain hip flexion using the reciprocal link, the hip joint and pelvis were pushing backward at the beginning of swing phase. This motion meant that the pelvis was moving in the wrong direction to participate in a ballistic swing, and it was considered that it might restrict users during the swing phase of gait.

Quantitative evidence for the reciprocal linkage mechanism between the hip joints of HKAFOs shows that the linkage benefits the user during stance phase in preventing bilateral hip flexion. However, the linkage does not appear to be used to drive swing hip flexion, and it is suggested by different methods of measurement that the linkage may restrict user efficiency during the swing phase of gait. In contrast to the quantitative evidence, a survey of qualitative descriptions of the function of the hip linkage mechanism almost universally emphasizes the positive function of the link during swing phase. The qualitative descriptions of the function of the reciprocal link are often cursory and can be contradictory.

It is clear that orthoses with reciprocally linked hips allow people to perform reciprocal gait at a similar level to alternative systems. However, in the search for improved systems, the mechanical function of the reciprocal link should be understood, along with the actual benefits and problems that its use incurs.