Tissue Pressure Tolerance As a Guide to WristHand Orthosis Design
Duane Toderan, C.O. * Tom Lunsford, M.S.E. *
An individual with paralyzed wrist extensor musculature presents a challenge to orthotists. When the wrist is inadvertently maintained in a flexed position, wrist, hand, and finger flexion contracture deformities may result. The weight of the hand itself, and the mechanical and orthotic limitations present formidable problems.
The orthotic recommendation for a patient with paralyzed wrist extensors is often a static wristhand orthosis which supports the patient's wrist against flexion while maintaining an extension angle at the wrist, opposition of the thumb, and curvature of palm.
A successful wristhand orthosis is not only one with a good appearance, but more importantly, fits well. A wristhand orthosis which fits improperly can create excessive pressure.
The forearm, wrist, and hand can be treated as a simple firstclass lever system with a force and lever on each side of a fulcrum. This lever system allows an analysis of the demand torque (hand) and support torque (orthosis). Then it follows that force can be deduced from the torque and pressure estimated by the ratio of force to associated area over which the force is applied. Lastly, the estimated values for pressure can be compared to tissue tolerance values and excesses identified.
In a normally extended wrist (30 deg.), the flexion demand torque must be equal to the extension support torque as shown in Fig. 1 .
Equ. 1
Where,
 F_{E} =
 extensor musculature force required to keep wrist extended lbs.
 d_{F} =
 extensor lever arm from extensor tendon midline to wrist joint axis n inches.
 W_{H} =
 weight of hand producing wrist
 d_{H} =
 wrist flexion lbs. hand lever arm from center of gravity of hand to wrist joint axis n inches.
If the extensor tendons were cut or denervated, the pronated hand would inevitably fall into flexion as shown in Fig. 2 . Without support extensor torque, an alternative (orthotic) form of support is required.
In most cases the paralyzed wrist can be supported with a static wristhand orthosis. As shown in Fig. 3 , the hand is supported with palmar and forearm components.
Experience with static wristhand orthoses suggests that there are three areas where pressure may become excessive. As shown in Fig. 4, these areas include the volar forearm, P1, palmar, P2, and dorsal wrist, P3 areas.
Volar Forearm Pressure (P_{1})
To determine the volar forearm pressure, P1, it is necessary to consider the force, F1, between the proximal strap and volar forearm surface as shown in Fig. 5.
The hand in Fig. 5 tends toward flexion producing a demand torque (T_{D}) given by Equ. 2
The corresponding support torque (T_{s}) afforded by the orthosis is given by Equ. 3
Where,
 F_{1} =
 proximal volar forearm strap force n lbs.
 d_{F} =
 length of forearm piece from proximal strap to wrist joint axis n inches.
When the wrist is adequately supported the demand torque is balanced by the support torque or Equ. 4
Rearranging equation (4) yields the following for the volar forearm force, Equ. 5
The volar forearm strap area is Equ. 6
Where,
 W_{S} =
 strap width n inches
 L_{S} =
 strap length contacting volar surface n inches Since mechanical pressure is defined as the ratio of force to area, the volar forearm surface pressure, P1, is obtained from equations (5) and (6).
Equ. 7
The numerator of equation (7) contains hand weight (W_{H}) and its lever, (dH) neither of which can be changed by the orthotist. However, the factors in the denominator forearm lever (d_{F}), strap width (W_{s}) and strap length (L_{S}.) can be changed to reduce the volar forearm pressure, P_{1}
The value of equation (7) to the orthotist can be realized by a practical example. Consider patients with hands that weigh 1 to 2 pounds (W_{H}) and each has a 3inch lever arm (d_{H}) from the center of gravity of the hand to wrist joint axis. Assume the area of the volar forearm strap is 1/2 wide (W_{S}) by 3 inches long (L_{S}). The resulting volar forearm pressure produced as a function of the forearm lever is shown in Fig. 6.
Before a value for forearm lever, d_{F}, can be selected from this graph, it is necessary to know the maximum allowable value for volar forearm pressure, P_{1}. The maximum volar forearm pressure can be obtained from the tissue tolerance curve shown in Fig. 7.
Tissue tolerance can be expressed as a specific pressure applied for a maximum period of time. Any combination of applied pressure and length of time above the curve is unacceptable; values below the curve acceptable. According to this relationship for "pressure tolerance," it is necessary that less than 30 mmHG pressure exists before the orthosis can be worn continuously (8 hours or longer). Referring to the curve of P_{1} versus d_{F}, shown in Fig. 6 , 30 mmHG requires a minimum 6inch proximal forearm lever. Therefore, the distance from the proximal extension of the forearm piece to the wrist joint axis should not be less than 6 inches to ensure that excessive pressure does not exist.
Palmar Pressure (P_{2})
The palmar pressure, P_{2}, is due entirely to the ratio of the weight of the hand (WH) and to the area of the orthosis in the region of the palm.Equ. 8
The area of the orthosis at the palm is given by (see Fig. 8 ).
Equ. 9
Where,
 W_{PE} =
 width of palmar extension inches
 L_{PE} =
 length of palmar extension inches
Substituting equation (9) into (8) yields Equ. 10
This relationship for palmar pressure is shown graphically in Fig. 9. In this case the palmar pressure, P_{2}, is compared with the width of the palmar extension, W_{PE}. The length of palmar extension, L_{PE}, in contact with the hand is assumed to be 3 inches. With the restraint that the pressure shall not exceed 30 mmHG continuously, the minimum width of the palmar extension is 0.7 inch as shown in Fig. 9.
The size of palmar extension (3 x 0.7 inches) will ensure safe pressure but may interfere with rotation of the metacarpalphalangeal joint. The distal edge of the palmar extension must be positioned proximal to the imaginary line formed by the second and fifth palmar creases.
Dorsal Pressure (P_{3})
The dorsal pressure, P_{3}, is represented in Fig. 10. This pressure is given by the ratio of the force, F_{3}, to the area of the dorsal piece of the wristhand orthosis making contact with the wrist.Equ. 11
The force F_{3} is an equal and opposite reaction to the sum of the forces of the hand, WH, and forearm piece, F_{1}.Equ. 12
The area of the dorsal piece is given by its length (L_{DP}) times width or W_{DP}.Equ. 13
Substituting equation (12) and (13) into (11) yields the following expression for dorsal pressure Equ. 14
Fig. 11 is a plot of dorsal pressure (P_{3}) versus the width of the dorsal piece (W_{DP}). In this case the hand lever selected is 3 inches, the forearm lever is 6 inches, and the length of the dorsal piece making contact with the wrist is assumed to be 2 inches.
For the dorsal pressure not to exceed 30 mmHG, the width of the dorsal piece must be a minimum of one inch.
Summary
Three areas of pressure produced by a static wristhand orthosis have been identified. The concept of demand torque produced by a paralyzed and flexible wrist and support torque provided by a static wristhand orthosis was introduced. The forces and levers of the anatomical demand system were compared to those of the orthotic supporting system. This information together with an estimate of the area over which the supporting forces occur allowed an estimate to be made of the pressure applied. The applied pressure was evaluated for tissue tolerance.
