Of the 1.6 million persons living in the United States with limb loss, approximately 1.3 million (86%) have an amputation of the lower limb.1,2 Of these, estimates suggest that 28% or 378,000 individuals have transtibial amputation.3
The socket has long been described by lower-limb prosthesis users as the most important consideration in their satisfaction with a lower-limb prosthesis.1,4,5 Its purpose is to transfer loads under both static and dynamic conditions with minimal movement between the limb and the prosthesis. Pressure distribution within the socket is an essential component to the comfort and function of the individual using a transtibial prosthesis. Ill-fitting sockets can lead to dermatologic concerns, injury to the limb, and decreased prosthetic utilization. The functional comfort of a transtibial prosthesis is dependent upon a number of interrelated factors including socket type and fit, interface materials, and suspension approaches.
For many decades, the standard of care for transtibial sockets was the joint-and-corset model in which the load-bearing forces were split between the socket and a thigh corset. Confining weight-bearing forces to the transtibial socket was not commonly performed until the introduction and popular acceptance of the patellar tendon bearing (PTB) socket in the 1950s.6 This approach is characterized by localized loading in load-tolerant areas and localized reliefs in areas that are generally intolerant to localized pressure. In the 1990s, the total surface bearing (TSB) socket was introduced in which the entire surface of the transtibial limb is used for load bearing.7,8 The efficacy of the latter appears to be enhanced by the integration of viscoelastic interface liners fabricated from a range of elastic materials including silicone, urethane, and other gel-like substances.9,10 Modern prosthetic sockets are generally a combination of PTB and TSB principles.11
For many years, the interface options worn between the limb and the socket were confined to fabric fitting socks and various foam materials. More recently, the viscoelastic materials mentioned previously have been exploited for their cushioning and ability to integrate suspension. As the interface materials in modern transtibial prosthetic solutions are often integral with suspension of the device, it is difficult to examine these two variables discretely.
With the introduction of the PTB socket design, a number of anatomically based suspension mechanisms were used, including supracondylar socket contours and cuff-strap suspension. The introduction of the TSB socket was largely concurrent with the development of silicone suction suspension and the Icelandic roll-on silicone socket (ICEROSS).9,10 Modern suspension systems connect the residual limb to the socket using a locking silicone liner with a distal pin-lock mechanism, through the creation of a suction environment by internal or external sealing sleeves or rings and through active vacuum where the negative pressure within the suction socket is actively elevated and maintained by an external vacuum component.12
After transtibial amputation, successful prosthetic rehabilitation requires the development of a comprehensive socket strategy inclusive of a socket design, interface materials, and suspension strategy. The most common variants in these design elements are described in Table 1.
Clinical practice guidelines (CPGs) are increasingly common in health care, with the Federal Agency for Healthcare Research and Quality (AHRQ) now housing over 1700 practice guidelines in its National Guideline Clearinghouse.13 Yet, the field of orthotics and prosthetics is underrepresented in this area, with only a single CPG listed in the AHRQ database. Encouragingly, the field has begun to develop and publish practice guidelines across a range of care episodes including the management of plagiocephaly,14 postoperative care following transtibial amputation,15 prosthetic foot selection for individuals with lower-limb amputation,16 prescribing guidelines for microprocessor-controlled prosthetic knees in South East England,17 and a two-part “Dutch evidence-based guidelines of amputation and prosthetics of the lower extremity.”18,19
The scope and depth of CPGs is variable, with direct implications on their resultant clinical relevance and ultimate incorporation into practice. The current effort is modeled after the CPGs of the American College of Physicians,20 with necessary adaptations to accommodate the emerging evidence base of orthotic and prosthetic care. The stated goals of this approach are to “provide clinicians with clinical-based guidelines based upon the best available evidence; to make recommendations on the basis of that evidence; to inform clinicians of when there is no evidence; and finally, to help clinicians deliver the best health care possible.”20(pp194)
Clinical utility is of paramount importance in this effort, culminating in a small number of succinct, actionable evidence-based recommendations.21 Notably, within this framework, although the resultant CPGs represent a comprehensive overview of available literature, deficits in the available literature preclude CPGs within this framework from providing comprehensive clinical guidance.
The purpose of this guideline is to present the available evidence on definitive transtibial socket design options, interface considerations, and suspension variations. The target audience for this guideline includes prosthetists, surgeons, physicians, therapists, case managers, and policy makers. The target patient population comprises individuals with transtibial amputation due to traumatic, dysvascular, or alternate etiologies.
A Medline search was conducted on May 2, 2017, to locate published secondary knowledge sources of evidence statements within the published literature. The following search terms were used: ((prosth*) AND transtibial) AND (socket OR suspension OR interface) AND (systematic review OR meta-analysis). This search originally yielded 17 abstracts. Of these, 9 articles were identified as secondary knowledge sources (i.e., meta-analysis, systematic review, or scoping review) that synthesized published findings of primary knowledge related to transtibial socket design, interface, and suspension.3,11,22–28 Two additional publications meeting this standard were referenced within the selected literature and included for consideration and analysis.29,30 Collectively, the 11 identified publications included one systematic review and meta-analysis,3 nine systematic reviews,11,23–28 and a single scoping review.22
In more recent publications, where authors provided explicit evidence statements, these were extracted for subsequent synthesis (Table 2). If explicit evidence statements were not provided, well-supported narrative statements were extracted (Table 3). Statements were confined to areas of clinical utility. Extracted statements are summarized in Tables 2 and 3 and addressed the following key considerations:
- 1. Comparative effectiveness: With regard to socket design, PTB sockets were compared against TSB sockets. Frequently, these comparisons overlapped with interface comparisons, with foam liners (i.e., Pelite) used in conjunction with PTB sockets, whereas elastomeric liners were used in conjunction with TSB sockets. Suspension comparisons were variable but can be generalized as comparisons between anatomic suspension techniques, mechanical locking liner techniques, and suction techniques, with VAS representing an extreme form of suction that has been extensively studied.
- 2. Benefits of treatments: Potential benefits found in the extracted evidence statements relate to timeliness of prosthetic fitting, enhanced activity levels, patient satisfaction, reduced movement of the residual limb within the socket (i.e., enhanced suspension), mitigation of forces within the socket, stabilization of limb volume, improved comfort, and better gait symmetry.
- 3. Harms of treatments: Potential harms found in the extracted evidence statements relate to injury to the residual limb secondary to socket forces, discomfort, heat and perspiration, donning difficulties, and system maintenance requirements.
The extracted evidence statements from Tables 2 and 3 were subsequently synthesized into the six recommendations below, comprising the CPG.
Recommendation 1: The static and dynamic pressure distribution of the residual limb within the socket are essential considerations in patient comfort, function, and well-being.
The socket has consistently been described by lower-limb prosthesis users as the most important consideration in their satisfaction with a lower-limb prosthesis.1,4,5 When present, dissatisfaction with a transtibial prosthesis is frequently caused by strains, injuries, and discomfort associated with fit of the residual limb within the socket.11 Research has shown that interface liners can help distribute loading and reduce pain.29 The challenge of maintaining a congruent fit is exacerbated by changes to the residual limb that can be difficult to predict or control.11 In addition to the immediate impacts of socket fit on the user’s comfort and the health of the residual limb, the fit of the socket over the limb affects biomechanical variables of function and performance.11
Recommendation 2: Total surface bearing sockets are indicated to decrease fitting times and enable higher activity levels.
Compared with PTB sockets, TSB sockets may lead to greater activity levels, fewer pressure problems, and improved satisfaction among active prosthesis users,25 a finding epitomized in the work of Yigiter et al.31 who cite increased gait symmetry, velocity, cadence, and balance with TSB sockets. This statement is made with a recognition that, within the published evidence, TSB sockets have generally been fabricated over elastic liner interfaces, whereas PTB sockets have generally been fabricated over foam interfaces, and the respective contributions of socket design, interface, and suspension cannot be reasonably inferred from current literature. The generalized statement of the benefits of TSB sockets is countered by the observations of a single small trial of 13 subjects who reported increased wearing time and activity with PTB sockets compared with TSB sockets, an observation similarly confounded by the different interfaces and suspension methods used with each socket design type.32
From an economic standpoint, although PTB sockets have a lower initial cost, this is offset by the need for patients to spend up to three times longer in the fitting process to achieve a satisfactory socket fit,22 observations supported by two unrelated clinical trials.33,34 Highsmith et al.22 summarized that these additional clinic visits, which require increased time commitments and travel costs as well as the risk of potential complications, ultimately increase latent costs of PTB sockets. Conversely, TSB sockets have a higher initial procurement cost, but result in fewer associated visits.22
Recommendation 3: Compared with traditional foam-based interfaces, viscoelastic interface lines are indicated to decrease dependence on walking aids, improve suspension, improve load distribution, decrease pain, and increase comfort.
Systematic reviews of existing clinical studies suggest viscoelastic interface liners offer clinical improvements relative to traditional PTB socket interfaces (i.e., foam and fabric). These improvements include decreased reliance on walking aids, improved load distributions against the residual limb, decreased pain, and increased comfort.3,30 These benefits were largely identified in early clinical trials with modest to large study populations of 27 to 83 subjects and included improved walking function,35–37 decreased reliance on upper-limb walking aids,33,36 decreased pain,37,38 and improved comfort.36–38
These benefits are offset by reports of difficulties in donning and doffing experienced by some users27 and a temporary increase in perspiration relative to that experienced within foam and fabric interfaces.27 The relative difficulty of donning viscoelastic interface liners has been inconsistent with early trials citing mixed user experiences.35,37,38 Temporary increases in perspiration with elastic liners has been reported with greater regularity.25,35,37,38 Reviewers have been critical of the quality of research related to transtibial liners and suspension, summarizing that there is insufficient research on user experience to inform specific interface recommendations.24,29,30
Recommendation 4: Among modern suspension options, vacuum-assisted suspension (VAS) sockets permit the least amount of pistoning within the socket, followed by suction suspension, and then pin-lock suspension. The traditional suspension options of supracondylar, cuff, and sleeve suspension provide comparatively compromised suspension.
Kahle et al. described agreement between two high-quality level 2 studies39,40 and one low-quality study,41 offering grade B evidence that VAS reduces movement of the residual limb within the socket.28 When study findings are aggregated, VAS sockets allow the least amount of movement between the residual limb and the socket. Progressively greater socket displacement is experienced with suction sockets without external vacuum assistance and locking liner suspension.26,27 These modern systems provide improved suspension relative to the historical standards of sleeve suspension and supracondylar suspension.3,26 However, because of the additional considerations associated with suspension variants, there is no single suspension system currently viewed as the standard for all individuals with transtibial amputation.27
Recommendation 5: VAS sockets are indicated to decrease daily limb volume changes while facilitating more favorable pressure distribution during gait.
VAS sockets have been studied extensively in recent years, culminating in a well-defined set of potential benefits associated with this socket-suspension system. These benefits include decreases in daily volume changes and in the peak positive pressures experienced during stance,23,28 as well as the maintenance of a better socket fit, improved mobility, comfort, stability, proprioception, overall satisfaction, prosthetic function, and quality of life.23
In their systematic review on VAS sockets, Kahle et al. asserted that VAS sockets increase volume, whereas non-VAS sockets reduce residual limb volume, citing two studies representing level 2 evidence.28 This finding was also asserted in other systematic reviews.3,26,27 This tendency toward limb volume increases is thought to occur as a result of increased negative pressures during swing phase in VAS prostheses.42,43
Kahle et al. also evidenced two high-quality studies41,42 representing level 2 evidence that VAS sockets favorably affect pressure distribution on the residual limb,28 an observation echoed by a similar level 2 evidence statement by Highsmith et al.3 and found in the assertions of Safari et al. that VAS sockets seem to affect residual limb health positively compared with other socket designs and that VAS sockets “seem to improve gait symmetry more than other socket designs.”26
The effect of VAS sockets on the associated constructs of mobility, balance, and function has been viewed differently by different reviewers. Gholizedeh et al.23 asserted that VAS socket systems improve mobility. Kahle et al. also cite two level 2 studies demonstrating improvements in the specific functional areas of spatiotemporal gait39 and balance confidence,44 but also note a third level 2 study reporting a decrease in activity with the use of VAS sockets.40
The relationship between favorable pressure distribution, skin irritation, and wound healing has been viewed differently by different authors. Although”” Gholizedeh suggested that “vacuum systems may reduce skin irritation, reduce pain, and assist in wound healing,”23 Kahle et al. asserted that, “…no evidence exists to support the notion that a prosthesis can assist in healing wounds with or without VAS.”28 Synthesis between these two views is found in a recognition that reduced pistoning reasonably reduces shearing forces which, in turn, reduces the incidence of skin perturbation and pain. Arresting the movement of the limb within the socket may reduce irritation over both healthy and ulcerated tissues, permitting granulation and healing of existing wounds.28
Recommendation 6: VAS sockets require both awareness and compliance on the part of the end user and are not universally indicated.
Despite the established benefits associated with VAS, it is not universally indicated. Gholizedeh et al. summarized succinctly that “…some patients prefer pin-lock systems to vacuum…,” whereas others reduce their activity in VAS.23 It has been suggested that the attention and skills needed for donning a VAS prosthesis may be difficult for some individual with amputation.23 In support of this contention, authors have reported upon the possibility of VAS systems creating skin blisters when worn improperly.40,45 Consistent with these reports, subject matter expert consensus affirms that vacuum systems require both careful clinical evaluation, can cause blistering if not worn properly, and require that the patient has sufficient cognitive ability to know what to watch for and how to fix problems.23 Further, it has been observed VAS requires more maintenance than other suspension systems,23 a reality that should also be weighed when considering its use.
INCONCLUSIVE AREAS OF EVIDENCE
Although the recommendations above summarize the best available evidence, limitations to this evidence base result in several areas of inconclusive findings. Foremost among these is that the related elements of socket design, interface materials, and suspension are largely integral to one another, precluding a precise understanding of their individual contributions to comfort and function.
Note: Clinical practice guidelines are “guides” only and may not apply to all patients and all clinical situations. Thus, they are not intended to replace clinical judgment but rather to supplement clinical practice decision making.
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