Surface Pressures in Lower Extremity Splints: A Biomechanical Study

Abstract

Background:

Though ubiquitously used in orthopaedic trauma, lower extremity splints may have associated iatrogenic risk of morbidity. Although clinicians pad bony prominences to minimize skin pressure, the effect of joint position on skin pressure and, more specifically, changing joint position, is understudied. The purpose of this biomechanical study is to determine the effect of various short-leg splint application techniques on anterior ankle surface pressure in the development of iatrogenic skin pressure ulcers.

Methods:

Various constructs of lower extremity, short-leg splints were applied to 3 healthy subjects (6 limbs total) with an underlying pressure transducer (Tekscan I-Scan system) on the skin surface centered on the tibialis anterior tendon at the level of the ankle. All subjects underwent anterior ankle surface pressure assessment when padding was applied in maximum plantar flexion and neutral position for conventional short-leg splints application in clinically relevant patient scenarios. Percentage change from initial contact pressure centered on the tibialis anterior with cast padding were calculated.

Results:

The percentage change in anterior ankle contact pressure when padding was applied in maximum plantar flexion (PF) and then definitively placed in neutral was increased at least 2-fold without the addition of plaster in lower extremity short-leg splints. Removing anterior ankle padding following final splint application in neutral reduced contact forces at the anterior ankle 46% and 59% in splints applied in maximum PF and neutral ankle position, respectively.

Conclusion:

The present study is the first of its kind to underscore and quantify clinically relevant technical pearls that can be useful in reducing risk of iatrogenic risk of skin breakdown at the anterior ankle when placing short-leg splints, mainly, that it is imperative to apply padding in the intended final splint position and to remove anterior ankle padding following splint application when able.

Level of Evidence:

Level IV, biomechanical study with clear hypothesis.

Introduction

Lower extremity splints are commonly used for stabilization as a temporizing measure, definitive management strategy for stable fractures, or immediately postoperatively following many orthopaedic procedures. However, their use is not without an associated risk (Figure 1). Poor splinting techniques are common, with one study demonstrating inappropriate splinting on 93% of patients.¹ Morbidity of poor splint technique can include undue pain, thermal injury, skin breakdown, and ulceration. Improper splinting necessitates replacement, adding a cost burden on hospital systems. Moreover, splint-related soft tissue complications are the second most common iatrogenic cause for referral to plastic surgery.¹⁴

Figure 1.

Anterior ankle wound following short-leg splint removal. Iatrogenic anterior ankle wound following improper short-leg splint application. The exact manner of splint application for this patient was not recorded.

Pressure ulcers result from sustained forces that compress the soft tissues between body mass, bony prominences, and the support surfaces underneath. Within subcutaneous tissue, vessels traverse in a parallel fashion to the skin surface. These structures can collapse easily from pressure from inadequate padding at bony prominences and/or poor contour of the splint materials. As a result, prolonged pressure above the threshold for capillary tolerance results in occluded blood and interstitial fluid flow which can ultimately go on to cause ischemia, pain, necrosis, and sloughing of dead tissue. Specifically, 32 mm Hg has been historically reported and classically cited as the minimum pressure to maintain patency of dermal skin vessels, and external pressures beyond this threshold will result in collapse,^13,15,16,18 causing thrombosis of the dermal vessels with subsequent damage. In canine muscle, capillary blood pressure is reported to be 25 mm Hg.¹⁰ It is generally recognized that healthy capillary pressure in human skin ranges from 20 to 40 mmHg, with 32 mmHg being considered the average.^2,15,17 In healthy patients, compressive loads prompt an accumulation of toxic metabolites that trigger edema formation, increased tissue perfusion, and associated acidosis. These tissue changes trigger afferent signals that elicit positional changes to relieve pressure and present further damage. Immobilized patients are unable to change position to alleviate areas of compression. Furthermore, these patients may be under significant analgesia, which may compromise protective sensations of pain associated with excess compression. Literature has suggested that ulcer formation can occur in as few as 2-6 hours following a compressive insult.¹² Thus, it is imperative that orthopaedic practitioners be aware of methods to reduce the likelihood of ulcer formation with immobilization.

Common orthopaedic practice is to pay special attention to padding of bony prominences such as the medial or lateral malleoli. However, skin flexion areas such as at the level of the tibiotalar joint in the dorsiflexed ankle splint can be subject to increased pressures from excessive folding of padding material, and the risk of skin pressure necrosis may be commonly overlooked and understudied.

The purpose of this biomechanical study is to investigate the effect of various short-leg splint application techniques on anterior ankle surface pressure that could result in the development of iatrogenic skin pressure ulcers in lower extremity splinting. More specifically, this study will examine the effect of ankle position while placing padding for the short-leg splint to evaluate the effect of mitigating increased contact pressures at the anterior ankle following an inappropriate splint application through removal of excess padding material. Our hypothesis is that maintenance of final foot position during initial application of skin padding will result in decreased contact pressures (compared to initial application in ankle plantar flexion) and, therefore, diminishing risk of iatrogenic skin complications. Conversely, placement of padding on an ankle in plantarflexion and then splinted in a neutral position will precipitate significant increases in anterior ankle contact pressure and may increase the risk of skin ulceration.

Methodology

Study Participants

Following ethics approval by our institutional review board, various constructs of lower extremity short-leg splints were applied to 3 healthy subjects (6 limbs total) with an underlying pressure transducer on the skin surface centered on the anterior ankle on the tibialis anterior tendon (Figure 2). Exclusion criteria consisted of subjects <18 years of age, subjects with existing foot or ankle pathology, significant skin pathology in the lower extremity, or participants who are deemed unsafe for kinematic analysis when wearing unilateral lower extremity immobilization secondary to a fall risk.

Figure 2.

Experimental setup: placement of Tekscan pressure transducer film. Tekscan transceiver suspended from proximal leg with pressure transducer attached and centered over the tibialis anterior tendon utilizing Tegaderm adhesive dressings to maintain centered position over the tibialis anterior tendon. Application of cast padding and splinting material follows, as described.

Anterior Ankle Surface Assessment

Surface pressure measurements were recorded using the Tekscan I-Scan system (Tekscan Inc, South Boston, MA) using the I-Scan 3000E sensor for all measurements. A sampling rate of 100 Hz was used for all trials. More specifically, using the Tekscan I-Scan system, the peak force over the tibialis anterior tendon localized at the anterior ankle was measured during different experimental conditions described in Tables 1 and 2. Peak forces (newtons) were measured along the tibialis anterior, and percentage changes from baseline conditions were reported.

Table 1.

Anterior Ankle Contact Pressures in Different Conditions.^a

	Increase in Peak Force at Neutral Dorsiflexion, %
	Subject 1	Subject 2	Subject 3	Subject 4	Subject 5	Subject 6	Mean; SEM
Condition 1: Padding only
Initial padding position
Maximum PF	323	106	53	478	163	308	328; 65
Neutral PF/DF (baseline)	—	—	—	—	—	—	—
Condition 2: Padding + plaster
Initial padding position
Maximum PF	304	240	321	482	217	343	318; 42
Neutral PF/DF	153	116	87	93	83	233	128; 24
Condition 3: Following removal of anterior ankle padding
Initial padding position
Maximum PF	61	97	108	35	64	134	83; 15
Neutral PF/DF	17	43	116	3	11.7	108	33; 16

Abbreviations: DF, dorsiflexion; PF, plantarflexion; SEM, standard error of the mean.

^aThis table highlights the increase in peak force at the ankle when components of a short-leg splint are placed in maximum compared to the preferred method of neutral ankle position under 3 conditions: (1) padding, (2) padding and plaster, and (3) removal of padding from the anterior ankle.

Table 2.

The Effect of Removing Anterior Ankle Padding Following Splint Application.^a

	Decreased Peak Force After Splint Application, %
Initial padding position	Subject 1	Subject 2	Subject 3	Subject 4	Subject 5	Subject 6	Mean; SEM
Maximum PF	40	58	49	23	52	53	46; 5
Neutral PF/DF	46	66	62	53	61	62	59; 3

Abbreviations: DF, dorsiflexion; PF, plantarflexion; SEM, standard error of the mean.

^aThis table compares the decrease in peak force at the ankle following the removal of anterior ankle padding when leg is splinted in maximum plantarflexion compared to neutral ankle position.

Three trials were collected for each subject, and the averages and standard deviations of those contributed toward a study sample average and SEM (Table 1). Sensors were centered on the tibialis anterior tendon at the level of the ankle and verified manually prior to data collection. Sensors remained adhered in the proper position over the tibialis anterior via Tegaderm transparent medical dressing (3M, Saint Paul, MN). Before testing, the pressure sensor was calibrated with a known isotropic square weight of 8 pounds that was felt to exceed the limit of loads experienced in this study, and the entire matrix area of the sensor was loaded to ensure precise calibration.

Experimental Groups

All subjects underwent anterior ankle surface pressure assessment according to Table 1. First, a contact pressure assessment was conducted without any padding applied to quantify experimental noise, which can be later subtracted from experimental conditions. Four-layers of Webril padding (Covidien/Medtronic, Dublin, Ireland) were then applied circumferentially to ankles in either maximum plantarflexion or neutral position with measurements taken at a neutral ankle position before and after plaster material application encompassing a standard short-leg splint consisting of a “U-slab” and a posterior slab of plaster of Paris splints. For our study protocol, the U-slab was applied first, and the posterior slab was then applied for all subjects. Final splints were wrapped with ACE bandage in standard fashion. All measurements were captured at the final splint foot/ankle position of neutral dorsiflexion/plantarflexion (DF/PF) (90°). Neutral position of the foot and ankle were confirmed with goniometer. Following measurements on completed splints, anterior contact pressures were reexamined following discretionary removal of approximately 50% anterior ankle padding that did not compromise splint integrity and may reasonably be conducted clinically. As an additional point of caution, it should be noted that if the magnitude of difference in contact pressures between two adjacent areas is too large, shearing of the dermis and inadvertent propagation of blisters may occur. As such, care should be taken to avoid removal of an excessive amount of the anterior ankle padding.

Statistical Considerations

Pressure between the bony prominence and external surface occludes the capillaries. Normal reported capillary pressure ranges from 24.5 ± 8.5 mm Hg in different segments (midpoint 24.5 mmHg).² External pressure of more than 33 mm Hg occludes the blood vessel so that the underlying and surrounding tissues become anoxic and if the pressure continues for a critical duration, cell death will occur, resulting in soft tissue necrosis and eventual ulceration.² Although an a priori power analysis using a conservative surface pressure to account for tenuous skin in the setting of lower extremity trauma originating from the basic science literature was attempted, it was felt that the overall clinical usefulness in providing descriptive statistics to reinforce a clinically relevant, generalizable topic for orthopaedic and nonorthopaedic practitioners who initially manage lower extremity trauma outweighed the practical utility of quantitative analysis to fundamentally communicate the same conclusion.

Results

The percentage change in anterior ankle peak force in testing conditions are described in Table 1 and Figure 3. Forces at the anterior ankle entered on the tibialis anterior were shown to increase an average of 328% (range, 53%-487%; SEM: 65) when splint padding material was applied in maximum plantarflexion and then positioned in neutral dorsiflexion without plaster. This trend was also seen when plaster was applied to all samples, with an average 318% (range, 240%-482%; SEM: 42) increase in peak force at anterior ankle compared to baseline when plaster was applied with a final neutral ankle position.

Figure 3.

Percentage change in anterior ankle forces when padding is applied in maximum plantar flexion and then placed in neutral. This figure highlights the increase in peak force at the ankle when components of a short-leg splint are placed in maximum plantarflexion and then placed in neutral ankle position under 3 conditions: (1) padding, (2) padding and plaster, and (3) removal of padding from the anterior ankle.

The effect of removing anterior ankle padding following splint application is described in Table 2 and Figure 4. Removing anterior ankle padding following final splint application reduced contact forces at the anterior ankle. The average percentage decrease in peak force after splint application was 46% (range, 23%-58%) and 59% (range, 46%-66%) in splints applied in maximum plantarflexion and neutral ankle position, respectively.

Figure 4.

The effect of removing anterior ankle padding following splint application: percentage decrease in peak force at anterior ankle. This figure demonstrates the decrease in peak force at the ankle following the removal of anterior ankle padding when leg is splinted in maximum plantarflexion compared to neutral ankle position.

Discussion

To our knowledge, this is the first study to investigate how surface pressures experienced at the anterior ankle are altered in relation to the position of the ankle during standard short-leg splint application. Pressures of the anterior ankle were shown to increase >200% when splint padding material is applied in maximum plantarflexion and then finally positioned in neutral dorsiflexion, irrespective of plaster application, suggesting that plaster application may play only a marginal role in anterior surface pressures. More importantly, these results indicate that inattentive splint padding placement may substantially increase the risk for the development of iatrogenic soft tissue complications. Therefore, splint padding should be applied with the foot positioned as close as possible to the final intended position on hardening of plaster material.

Our study also provides evidence that a decrease in peak pressures at the anterior ankle is observed when anterior ankle padding is removed after the plaster has hardened. Thus, a secondary, but important, clinically relevant point that can be garnered from this study is that in settings where splint padding is applied in suboptimal foot position before the splint position is finalized, removing anterior ankle padding may significantly decrease peak pressures, which may subject a patient to anterior ankle skin risk. It should be noted, though, that if the difference in contact pressures between 2 adjacent areas is too different, this may result in shearing of the dermis and inadvertently propagate blister formation. As such, too liberal removal of anterior ankle padding should be avoided.

Although the pathophysiology of ulcer formation following splint immobilization is well understood,^7,16,19,20 there is no literature investigating the effect of ankle position during application of splint padding on surface pressures of the anterior ankle. We know the most at-risk population for developing pressure ulcers following immobilization are patients with decreased sensation as from diabetic neuropathy and those with developmental delay and spasticity. In the trauma scenario, patients with multiple injured extremities and those in an obtunded or comatose state may be particularly at risk.⁹ Furthermore, the accumulation of toxic metabolites initiated by compressive loads, seen with splint application that mediates edema formation and increases tissue perfusion; compensatory acidosis and tissue necrosis is amplified with soft tissue injury associated with lower extremity trauma. Barry et al³ assessed predictive factors for pressure ulcer formation in the pediatric population and discovered that younger age, decreased weight, increased time from injury to orthopaedic specialist follow-up, and increased number of days in a splint were all correlated with an increased incidence of pressure ulcer formation. These authors found that a 1-day increase from the time of injury and immobilization to specialist follow-up increased the risk of developing a pressure ulcer by 18%. They also noted a higher incidence of iatrogenic pressure injuries if the extremity was immobilized at a rural emergency department or urgent care compared with an academic hospital, implying that the lower volume of fractures, availability of cast or splint technicians, and lack of knowledge of splinting and casting principles at a smaller facility may play a role in pressure ulcer formation.

Our study’s findings provide evidence that the ankle should be held in the desired final position during the application of splint padding to minimize the peak surface pressures of the ankle both after the plaster is applied and when padding is removed from the anterior ankle. This decrease in sustained anterior ankle surface pressure presumably reduces shear forces and local capillary pressure allowing increased perfusion of local tissues and decreased soft tissue complications. We believe this is a matter of patient safety primarily. In addition, given that the average cost of treatment for a stage IV pressure ulcer and its downstream complications is approximately $130000, it is economically advantageous to educate orthopaedic surgeons, emergency department physicians, splint and cast technicians, and urgent care staff of these findings for the prevention of these injuries.¹¹

Our study is not without limitations. The greatest limitation of our study was our small sample size, which intrinsically should warrant readers to interpret the quantitative data reported with a degree of caution. Although only 6 limbs were reported in this study, our authors believe that the findings reported reinforce a clinically relevant, generalizable topic for orthopaedic and nonorthopaedic practitioners who initially manage lower extremity trauma. Additionally, there is relatively no risk in using these findings for future splint application to decrease the morbidity and cost associated with iatrogenic pressure injuries following lower extremity immobilization. Future research should aim to build on our preliminary work and contribute larger samples to the literature. Also, Tekscan systems are designed to be used for measurements on flat surfaces; when carefully positioned in anatomical areas of interest, they have been validated for many orthopaedic applications.^4,5,8 Although the ankle joint is not a flat surface, we subtracted a baseline “noise” measurement from the recorded measurement during splint application to obtain the most accurate value. Morphology mismatches between anterior ankle skin and contact pressure sensors may increase variability seen in our results. This device has also been implicated in other studies evaluating dynamic contact mechanisms and surface pressures in other parts of the body with low inter-observer variability.^6,12

Conclusion

The present study is the first to quantify clinically relevant technical pearls that can be useful in reducing the risk of iatrogenic risk of skin breakdown at the anterior ankle when placing short-leg splints. It is imperative to apply padding in the intended final splint position and to remove anterior ankle padding following splint application when able. Future research should aim to expand on these results by further investigating factors that may mitigate risk of skin ulceration after placement of short-leg splints, especially in patients at heightened risk for this complication given neurologic status or lengthened period of immobilization.