08 - Biomechanics.pdf

8/11/2019 08 - Biomechanics.pdf

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Biomechanics of the Wrist Joint

The wrist complex is biaxial joint, with motions offlexion/extension(volar flexion/dorsiflexion) around a coronal axis, and radial deviation/

ulnar deviation (abduction/adduction) around an anteroposterior axis.[24]

In the normal wrist, the total arc of motion from full flexion to full

extension is approximately 150.[24]This motion is made up approximately

equally by motion at the midcarpal and radiocarpal joints. However, the

midcarpal joint contributes more to flexion (62%) than does the radiocarpal

joint as the wrist moves from neutral to full flexion. Conversely, as the wrist

moves from neutral to full extension, the radiocarpal joint contributes more

(62%) than the midcarpal joint.[25] Further, wrist radial-ulnar deviation is

contributed to by motion at the midcarpal and radiocarpal joints, with the

majority (55%) of this motion occurring at the midcarpal joint.[26]

As the wrist moves from radial to ulnar deviation, the proximal row

extends as well as deviates ulnarly. As the wrist moves from ulnar to radial

deviation, the proximal row flexes and deviates radially. The distal row also

translates dorsally in ulnar deviation and volarly in radial deviation. This

translation may be the cause of proximal row extension and flexion.[27]


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(Fig. 5) A, In radial deviation, the proximal carpal row deviates toward the radius, translates toward the

ulna, and flexes as seen by visualizing the lunate on the lateral radiograph. B, With the wrist in neutral, the

capitate, lunate, and radius are nearly colinear. C, In ulnar deviation, the proximal row deviates toward the

ulna, translates toward the radius, and extends as visualized by the lunate on the lateral radiograph. [28]


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Biomechanics of F ractur e Reduction

Traction, ligamentotaxis, periosteotaxis, and manipulation are the

mainstays of fracture reduction. The brachioradialis is the only muscle

attached to the distal radial fracture fragment. Sarmiento and colleagues [29]

recognized the resistance and deforming force of the brachioradialis on the

distal radial metaphyseal or styloid fragment during the wrist flexion and

forearm pronation maneuvers of classically applied closed reduction

techniques. The brachioradialis also may remain a deforming force after

closed fracture reduction. They also reported and advocated fracture

reduction, positioning, and cast bracing with the forearm in a supinated

position to relax brachioradialis tension during and after fracture

reduction.[29]

The rule of the majority, also known as the vassal rule, may be

helpful in assembling the fracture fragments. This rule states that the major

fragments should be realigned, and that the smaller or vassal fragments

follow the major fragments into position. Replacement of each of thearticular fragment components before definitive plate fixation may avoid

some of the difficulties that may be encountered in reducing ulnar die-

punch fragments after radial styloid fixation. Fluoroscopy or arthroscopy or

both may be useful in achieving fracture and articular alignment. Kirschner

wires may be used for provisional fixation before plate insertion.[4]

Biomechanics of plate of the distal radius

Plate strength is proportionate to the cube of its thickness and

inversely proportionate to the cube of its length.[30] Screws enhance plate

strength and holding power at the plate-bone interface. Wider spacing of

screws in the stem increases the bending strength of plate-screw-bone


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fixation. The torsional strength of plate stem fixation is independent of

screw spacing and is proportionate to the number of screws holding the

stem.[31]

F ixed-Angle Principle:

The working portion of a buttress plate is the bar - the distal

segment of the plate supporting the metaphyseal fracture fragment or

fragments. Support of the metaphyseal fragment and overall plate-bone

construct strength may be improved by blades affixed to the plates or screws

or pegs locked into the screw or peg holes of the bar by matching threads.

Each fixed-angle blade or locking screw or peg provides an additional point

of fixation within the plate and increases plate stability. [30] Fixed-angle

blades or locking screws or pegs in the bar of the plate provide additional

support for the articular surface of the distal radius against axial loads

compared with conventional screws.[32] Several plates have a fixed-angle

screw or peg option for the bar of the plate (Fig. 6). The increased stability

of fixed-angle blades or locking screws or pegs may be especially

advantageous in osteopenic bone.[33]

The distal volar plate (DVP) (Hand Innovations, Miami, FL) and

similarly designed plates combine fixed-angle locking screws or pegs in the

stem of the plate with robust design so that they may be applied to the

palmar side of the distal radius for almost all fracture configurations

regardless of the direction of instability (Fig. 7).[33]

The goal of this platedesign is consistently to avoid dorsal plate application and its consequences.

Fixed-angle pegs follow the articular contour, are directed to support the

articular surface, and help to ensure fixation of commonly found articular

fragments. The radial most pegs are directed into the styloid, and the ulnar


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most pegs are directed into the dorsal ulnar edge of the radius to incorporate

styloid and dorsal die-punch fragments. Failure to incorporate the dorsal

die-punch fragment may lead to loss of reduction and arthrosis. The distal

palmar edge of the plate supports palmar die-punch fractures, which alsomay be incorporated with pegs.[34]

(Fig. 6) A, Threaded standard screw. B, Partially threaded standard screw. C, Threaded locking screw. D,

Locking peg. Arrows pointing to C and D indicate a space between the locking plate and the bone.

Standard holes and flexible bushings in locking holes allow 15 degrees of screw angulation from the

perpendicular position. (Universal Distal Radius System; courtesy of Striker Leibinger Micro Implants,

Portage, MI.)[4]


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(Fig. 7) A, First-generation DVP plate. B, Undersurface first generation DVP plate with a row of locking

pegs (arrow in B) designed to parallel and support the subchondral portion of the articular surface of the

distal radius. C, Second-generation DVP plate. D, A proximal row of screws (arrow 1) or pegs (arrow 2)

may be inserted to incorporate or support the dorsal lip or fragments of the distal radius. (Courtesy of Hand

Innovations, Miami, FL.)[4]


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Locking Plate Stems and Combination Plate Holes

(Combiholes)

The fixed-angle principle also may be applied to the plate stem.Elliptical plate holes (combiholes) have been added to the stems of the

AO/ASIF distal radius locking plate set (Fig. 8) (Synthes, Paoli, PA).

Combiholes allow the option of inserting either a fixed-angle locking

screw or a conventional screw. Standard screws compress the plate onto the

bone and stabilize the fracture owing to friction between the plate and the

bone. Locking screws inserted into the stem of the plate provide an

additional point of fracture fixation, prevent screw toggle, and increase plate

resistance to axial loads compared with conventional screws, owing to

locking screw head thread engagement in corresponding threads within the

locking plate hole. Distal radius locking plates are precontoured and do not

have to be shaped to or rest flush on all parts of the bone and, in essence,

may act as an internal fixator (i.e., an implanted external fixator) (Fig. 9).

This feature makes locking plates more biocompatible with the bone. A

locking plate might be envisioned as the ultimate external fixator with the

plate (connecting bar) placed extremely close to the mechanical axis of the

bone, maximizing its stability. Locking plate stems may be especially

advantageous in osteopenic bone. [30]

The pullout strength of a unicortical screw from bone is about 60%

compared with a bicortical screw. The surgeon must decide whether to

engage one or both cortices. Unicortical drilling may minimize damage to

the endosteal circulation of the distal radius and eliminates the need to

measure screw length.[30]


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(Fig. 8)

A to C, Combihole (A) allows engagement of a conventional screw (B) or a locking screw (C).

Arrow 1, The smooth portion of the combihole accommodates a standard screw head. Arrow 2, The

threaded portion of the combiholeaccommodates a locking screw head. Arrow 3, Space between the

fixed-angle locking plate and the bone surface. Standard screw holes or bushings incorporated in locking

plate holes may allow a few degrees of angulation from the vertical position. (Courtesy of Synthes, Paoli,

PA.)[4]

(Fig. 9) Small fragment locking T-plate used as an internal fixator with a small space betweenparts of

the plate and the bone (arrows). (Courtesy of Synthes, Paoli, PA.) [4]


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Rationale and Basic Biomechanics:

Although the concept of volar plating could be initially attributed to

Lanz and Kron[35] back in 1976 for plate fixation after osteotomy of

malunited distal radius fractures, the volar approach remained restricted to

fixation of volar rim fractures in the acute setting only. [36]Volar plating was

first recommended for fixation of both typical and atypical distal radius

fractures by Georguoulis and associates in 1992.[37]This was published in a

little-known journal and was not widely accepted for dorsally displaced

fractures until the landmark paper by Orbay and Fernandez in 2002.[38]Volar

plating offers many advantages when used in dorsally displaced fractures.

The key to its success is to ensure that this was a locking plate, hence

creating a fixed-angle device that would maintain the reduction and

eliminate screw toggle (Fig. 10). Volar plating also provides the opportunity

to release the pronator quadratus muscle, which is often trapped in the

fracture and can be a cause of pronation contracture.[39]

A nonlocking plate when used in buttress mode can resist only

moderate axial and bending forces. Thus, a simple nonlocking volar plate

used in a dorsally displaced fracture without any bony contact in the

opposite cortex is subject to much higher axial and bending loads, leading to

failure. Therefore, a stable and strong volar fixation of a dorsally displaced

fracture is only possible with a fixed-angle locking plate that can resist such

high forces. Fixed-angle implants transfer load stress from the fixed distal

fragment to the intact radial shaft, thus enhancing peg/plate/bone construct

stability (Fig. 11), unlike rigid internal fixation devices that rely mainly on

the frictional force between plate and bone to achieve fixation.[39]


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(Fig. 10) Schematic diagram showing volar fixation maintaining the anatomy of the radius but screw toggle

leads to plate motion relative to the shaft, which can lead to late failure. [40]

(Fig. 11) Schematic diagram showing fixed-angle implant transferring load stress from the fixed distal

fragment to the proximal radial shaft.[40]


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The ideal volar implant should have a design compatible with the

volar articular surface of the radius and should provide concomitant angular

and axial stability while stabilizing the dorsal surface.[41] The distal volar

plate (DVR Hand Innovations, Depuy Orthopedics, Warsaw, Indiana) hastwo parallel rows, and the orientation planes of their respective pegs

specifically match the complex three-dimensional shape of the radial

articular surface.[40]

The primary row pegs are directed obliquely from proximal to

distal to support the dorsal aspect of the articular surface. They are angled

accurately to provide support for the radial styloid and the dorsal ulnarfragment. These pegs are most effective in supporting the dorsal aspect of

the subchondral plate and hence avoid the re-displacement of the dorsally

displaced fractures. Concurrently, their action induces a volar force that

tends to displace the fragments in a volar direction, an effect that must be

opposed by a properly configured volar buttressing surface.[40]

To enhance fracture fixation in cases of severe comminution,

volar instability, or osteoporosis, an additional row of pegs originating from

a more distal position on the plate and having an opposite inclination to the

proximal row was conceived. The distal row is directed in a relatively

proximal direction and crosses the proximal row at its midline and is

intended to support the more volar and central part of the subchondral bone.

It prevents the dorsal rotation of a volar marginal fragment and volar

rotation of severely osteoporotic or unstable distal fragments with central

articular comminution, thus neutralizing volar displacing forces of the pegs

in the proximal row.[40]


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The advantages of a volar exposure and plating include the following:

1. Dorsally displaced fractures are simpler to reduce because the volar

cortex is usually disrupted by a simple transverse line.

2.

Anatomic reduction of the volar cortex facilitates restoration of radial

length, radial inclination and volar tilt.

3. The avoidance of dissection dorsally helps to preserve the vascular

supply to the dorsal fragments.

4. Because the implant is separated from the flexor tendons by the

pronator quadratus, the incidence of flexor tendon complications is

lessened

5.

When stabilized with a fixed angle internal fixation device, shortening

and secondary displacement of articular fragments is improved, and the

need for bone grafting is reduced.[43]

Several studies have compared outcomes of dorsal versus volar

plating of distal radius fractures. Ruch and Papadonikolakis[45]performed a

retrospective review of 34 patients, 20 of whom had undergone dorsal

plating and 14 of whom had volar plating. The authors found that both

groups of patients had similar DASH scores, but the functional outcome in

terms of Gartland and Werley scores was better in the volar plating group. In

addition, there was a higher rate of volar collapse and late complications in

the dorsal plating group compared with the volar plating group.[45]

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08 - Biomechanics.pdf