Wrist function in malunion: Is the distal radius designed to retain function in the face of fracture?

Abstract

Introduction

Fractures of the distal radius are the most common fracture in humans and are the sempiternal hazard of 3.5 million years of bipedalism. Despite the antiquity of the injury, one of the most controversial topics in current orthopaedics is the management of distal radius fractures. It has been suggested that radiographic appearances rarely correlate with functional outcomes. As the success of the human species is predicated almost exclusively on its dexterity and intelligence, it is conceivable that the distal radius has evolved to preserve function even in the face of injury. We therefore hypothesise that the distal radius is designed to accommodate the possibility of fracture.

Methods

We conducted a review of studies comparing fracture pattern and form with function. We also explore the paleoanthropological evidence and comparative studies with other primates.

Findings

The evidence points to the human distal radius being highly tolerant of post-fracture deformity in terms of preservation of function. In addition, the distal radius appears to have apparently anatomically ‘redundant’ features that confer this capability. We believe these phenomena to be an evolved trait that developed with bipedalism, increasing the chances of survival for a species whose success depends upon its dexterity.

The Laetoli footprints, dated at 3.5 million years old, were discovered in Tanzania in 1978.¹ They belonged to human ancestor Australopithecus, of whom Lucy is the most famous.² The footprints are the earliest direct evidence of exclusive bipedalism. Bipedalism liberated hands for dexterous activity, which was germane to human success. With this locomotor advantage came the hazard of falling and injuring this valuable commodity. Unsurprisingly, distal radial fractures are the most common fracture in humans.³ In 1814, the Irish surgeon Abraham Colles described his eponymous fracture: “The limb will at some remote period again enjoy perfect freedom in all of its motions and be completely exempt from pain: the deformity, however, will remain undiminished through life.”⁴

In other words, Colles noted 200 years ago that the distal radius was resilient to the effects of fractures, with little disability even in the face of significant deformity. Human evolutionary success has been exclusively dependent upon intelligence and dexterity. It is therefore conceivable that the distal radius has evolved to accommodate the possibility of fracture by preserving function in the event of injury.

Anatomy, pathomechanics and adaptations

Distal radius fractures occur following a fall onto an outstretched hand.⁵ This produces the classical deformity, characterised by reduction of volar tilt and radial height. The distal radius appears to have developed a distinctive anatomical profile to counteract the deformities resulting from this fracture pattern.⁶ It is inclined 11° (range 4–22°) to the volar (palmar) surface. Its surface is also 22° inclined medially to the ulna. The radial height is the distance between two parallel lines, one co-linear with the scaphoid radial articular surface and the second tangential to the tip of the radial styloid. This measures 11.6 mm. The distal radial articular surface sits 0.6 mm (range –2 mm to 2 mm) proud of the ulna.

Biomechanical and in vivo studies

Pogue et al showed in cadaveric studies that 20° of dorsal or palmer angulation was required before pathological radio-carpal pressures developed.⁷ Patterson and Viegas observed that cadaveric wrists could tolerate changes in radial inclination, volar inclination and radial shortening of up to 20°, 38° and 6 mm, respectively, before pathological biomechanics resulted.⁸ Thus, the dimensions of the wrist appear to possess 20° of ‘redundant’ radial inclination, 38° of ‘redundant’ volar inclination and 6 mm of ’redundant’ length, which may compensate for fracture deformity.

Forward et al examined patients with distal radius malunion, with a mean and maximum age of 25 years and <40 years, respectively.⁹ After a mean follow-up of 38 years, the average radial dorsal tilt was 11°. This represents a deviation of 22° (range18°–25°) from the uninjured contralateral wrist. The cohort also had a mean loss of radial length of 3.5 mm and loss of radial inclination of 8°. The researchers found that functional scores were normal, with no clinically significant difference in objective or subjective function between injured and uninjured wrists, as measured by the Disability of the Shoulder, Arm and Hand (DASH) and Patient Evaluation Measure, respectively. They conclude: “We have not been able to identify any thresholds of malunion beyond which function was clearly worse as measured by the Patient Evaluation Measure and DASH scores”.

Brogren examined the effect of malunion-union on wrist function 1 year after wrist fracture, which showed that AO fracture type had no bearing on function.¹⁰ In further work, Brogen showed that only those with both dosrsal tilt >10° and ulnar variance >2 mm had some loss of function at 2 years’ follow-up. There was no difference between those with no malunion and those with one of either dorsal tilt >10° or ulnar variance >2 mm.¹¹

Gliatis examined the outcomes of 169 patients with distal radius fracture who had a mean age of 35 years at the time of injury and a mean follow-up of 4.9 years.¹² Neither radial height nor radial inclination influenced outcome, despite a variance in radial height of 19 mm to -11mm and in radial inclination of 31° to -24°. Those with dorsal tilt of beyond 10° did, however, suffer inferior outcomes, which included dorsal tilt angles of 10°–31°.

Intra-articular distal radius fractures

Knirk and Jupiter laid down the orthopaedic dogma that restoration of “articular congruity was the most critical factor in achieving a successful result” following wrist fracture.¹³ However, the paper has been challenged on methodological grounds.¹⁴ Catalano et al examined 26 fractures in patients aged under 46 years with mean follow-up of 7 years using computerised tomography and two observers.¹⁵ They concluded: “The functional status...did not correlate with the magnitude of the residual step and gap displacement at the time of fracture-healing. All patients had a good or excellent functional outcome irrespective of radiographic evidence of osteoarthritis of the radiocarpal or the distal radio-ulnar joint or non-union of the ulnar styloid process.” Similar results were reported by Leung.¹⁶

Gliatis et al noted that intra-articular steps reduced very fine dexterity but did not affect strength, work or daily activity, and did not predict pain.¹² His cohort also involved the young (mean age 35 years) who were followed-up for an average of 5 years. Tellingly, the size of the intra-articular step could not determine the outcome. In addition, articular surface gaps had no bearing on strength, dexterity or the degree of interference with work/daily activities. They concluded that intra-articular fractures were slightly more injurious than extra-articular fractures, but not considerably so.

Forward et al found the distal radius to be resistant to developing osteoarthritis.⁹ In their study of 106 patients aged under 40 years, distal radius malunion was defined as radial inclination <20°, radial shortening >2 mm and loss of volar tilt. At a minimum follow-up of 33 years, 67% of those with an extra-articular fracture had osteoarthritis that was no worse than that on the uninjured side. In those who suffered intra-articular fractures, this figure was 40%. The findings are significant given that, in antiquity, humans may not have survived long enough to develop function-limiting osteoarthritis.

Affects of wrist osteoarthritis

Knirk and Jupiter noted that, even in the presence of radio-carpal osteoarthritis, there was no reduction in grip strength.¹³ Similar findings have been reproduced by other authors.^17–19 Giannoudis et al performed a meta-analysis of evidence regarding the effect of traumatic articular steps on functional and radiographic outcome in a number of joints.²⁰ The evidence showed that the distal radius was highly tolerant of articular steps and osteoarthritis. Although steps >2 mm tended to cause osteoarthritis and inferior early outcomes, long term outcomes tended to be very good or excellent. The authors concluded: “Surprisingly, high levels of function are achieved despite radiographic evidence of deterioration of the radiocarpal joint.”

Fracture in children and possible adaptive features

Noonan and Price concluded in their review that children could tolerate 100% displacement of the distal radius, with up 20° of angulation, and still remodel to normal anatomy if the child had at least 2 years of growth remaining.¹² In children, the distal radius does not adopt the adult appearance until physeal fusion. Although the remodelling potential is lost once fusion occurs, it is possible that the adult volar tilt, radial height and radial inclination mitigate against the effects of subsequent fracture.

Comparison with primates

There are three modes of primate locomotion: bipedalism, knuckle-walking and brachiation (branch swinging). The distal radius of knuckle-walkers is characterised by a dorsal projection with a concavity in which the proximal carpal bones nestle.²² This is thought to allow the wrist locking to resist extension and facilitate knuckle-walking. The wrist of brachiators is deeply concave, adopting a ball (carpus) and socket (distal radius) morphology.²³ While this profile permits tool manufacture, use and projection, it has not been adopted with the bipedal Homo and ancestor. It is posited that, while this configuration is functionally useful given the range of movement it allows, the wrist would be incapacitated following fracture. Falling onto an outstretched wrist would lead to a die-punch injury in which the carpals impacts into the distal radius socket, causing it to explode, with numerous intra-articular fragments. For brachiators, a fall will be broken by branches, and there would therefore be no selective pressure to produce anatomical variants that could accommodate the possibility of distal radius fracture.

The evolution and origin of distal radius anatomy: Dexterity in antiquity

Anatomically, modern Homo sapiens appeared in Africa 250,000 years ago.²⁴Australopithecus, living 1–4 million years ago, is ancestor to the Homo species. It was exclusively bipedal but also spent significant periods in an arboreal milieu. The profile of the species’ distal radius is, however, similar to that of Homo sapiens.²⁵ The same is true of Ardipithecus, a putative Australopithecus ancestor and earliest Homo ancestor, which inhabited Africa around 4.4 million years ago.^25,26 It is thought to have been a facultative biped, walking erect but also scaling trees. The distal radius of Homo sapiens and our bipedal ancestors appears to have developed along an evolutionary stream distinct from that of the brachiator and knuckle-walker.²⁷ Both the brachiator and knuckle-walker profiles predated that of the biped, but the Homo genus and ancestors do not retain them. This is in spite of the ball and socket profile allowing skilled activity.²⁴ Hence, the Homo species distal radius profile appears linked to bipedalism.

Fracture would have expressed a strong selective pressure. Archaeological finds show that it was very a common injury amongst early Homo sapiens and Homo neanderthalensis. Every complete adult Neanderthal skeleton aged 25 years or older shows fractures and fracture healing.²⁸ Ortner et al reported that, after arthritis, fracture was the most common pathological finding in the skeletons of archaic humans.²⁹ A fracture rate of 7.1% has been recorded in the radii recovered from populations living in North America around 3,500 years ago.³⁰ Lovejoy and colleagues examined a similar population and observed that Colles type distal radius fracture was one of the most common injuries.³¹

Treatment of distal radius fractures

The oldest reported treated fractures date from 130,000 years ago in Croatia, when only Neanderthals inhabited Europe.³² However, there is, even today, no consensus on the optimum management of wrist fractures.⁶ The 2009 American Academy of Orthopaedic Surgeons (AAOS) guidelines recommend operative fixation post-manipulation if the fracture has >10° of dorsal angulation, >3 mm of radial shortening or an articular step >2 mm. However, the AAOS could not recommend any one fixation technique over another.³³ Moreover, a Cochrane review found no difference in functional outcome between operative methods.³⁴

The uncertainty regarding the optimum mode of fixation of distal radius fractures largely revolves around there being little correlation between the radiographic appearance functional performance.³⁵ This remains a consistent finding,^35–39 thus supporting the hypothesis that the distal radius has evolved to be tolerant of wrist fracture.

Conclusions

The concept of supra-normal adaptation with the development of physiological parameters in excess of that required to survive is not new: adults can lose up to 15% of blood volume before tachycardia develops,⁴⁰ while 70% of the small bowel can be removed before ‘short bowel syndrome’ results. Homo sapiens display a distinct distal radius anatomy that retains function in the face of deformity. The evolution of this anatomical configuration is possibly a skeletal example of supra-normal evolution.