Artikel Vacuum phenomenon in equine carpal , metacarpophalangeal and metatarsophalangeal joints

INTRODUCTION The vacuum phenomenon is recognised in joints in which stress distracts the opposing articular surfaces. It has been postulated that increasing the joint space creates a negative pressure that attracts gas from surrounding extracellular spaces. This gas then accumulates in the joint, filling the void created in the synovial fluid. Radiographically, gas has commonly been noted between human articular facets and intervertebral disc spaces associated with degenerative disc disease. Other sites in humans where the vacuum phenomenon has been seen include the shoulder, knee, hip, metacarpophalangeal 9 , sternoclavicular and sternocostal joints. The most widely recognised example of this phenomenon is secondary to the traction-induced vacuum seen in hyper-extended shoulders and hips in paediatric radiographs. In dogs the vacuum phenomenon has been described in the scapulo-humeral joint associated with osteochondrosis, in intervertebral discs associated with other radiographic features of degeneration, and in the sternum and coxofemoral joints. In horses this phenomenon has been described in the metatarsophalangeal joint of 2 horses, 1 with a subluxated joint, and 1 with a normal joint. During routine flexed radiographic studies of horses at the Veterinary Academic Hospital , University of Pretoria, focal radiolucencies have been seen occasionally in carpal and metatarsophalangeal joints. These were interpreted as representing vacuum phenomena. This study determines the radiographic incidence of the vacuum phenomenon in stress-flexed normal and pathological carpal, metacarpophalangeal and metatarsophalangeal joints.


INTRODUCTION
The vacuum phenomenon is recognised in joints in which stress distracts the opposing articular surfaces 13 .It has been postulated that increasing the joint space creates a negative pressure that attracts gas from surrounding extracellular spaces 9 .This gas then accumulates in the joint 2 , filling the void created in the synovial fluid 13 .
Radiographically, gas has commonly been noted between human articular facets and intervertebral disc spaces 4,8,12,13 associated with degenerative disc disease.Other sites in humans where the vacuum phenomenon has been seen include the shoulder 9 , knee 3 , hip 9,17 , metacarpophalangeal 1 9 , sternoclavicular and sternocostal joints 14 .The most widely recognised example of this phenomenon is secondary to the traction-induced vacuum seen in hyper-extended shoulders and hips in paediatric radiographs 9,10 .
In dogs the vacuum phenomenon has been described in the scapulo-humeral joint associated with osteochondrosis 16 , in intervertebral discs associated with other radiographic features of degeneration 5 , and in the sternum 18 and coxofemoral joints 7 .In horses this phenomenon has been described in the metatarsophalangeal joint of 2 horses, 1 with a subluxated joint, and 1 with a normal joint 15 .
During routine flexed radiographic studies of horses at the Veterinary Academic Hospital, University of Pretoria, focal radiolucencies have been seen occasionally in carpal and metatarsophalangeal joints.These were interpreted as representing vacuum phenomena.This study determines the radiographic incidence of the vacuum phenomenon in stress-flexed normal and pathological carpal, metacarpophalangeal and metatarsophalangeal joints.

MATERIALS AND METHODS
The incidence of vacuum phenomenon was established in horses with clinically normal joints, in joints with known pathology and in anaesthetised horses with joints under traction.
Part 1 of the study was to assess the incidence of the vacuum phenomenon in clinically normal horses.Survey radiographs of carpal, metacarpophalangeal and metatarsophalangeal joints were made.These joints were selected for the ease with which they can be radiographed and because the vacuum phenomenon has been described previously in the metatarsophalangeal joint of a horse 15 .Joints were included in the study only if no major radiographic abnormalities were present in standard lateromedial, dorsopalmar (plantar), dorsolateral-palmaro (plantaro) medial oblique, dorsomedial-palmaro (plantaro) lateral oblique and flexed lateromedial views of the relative joints of both left and right limbs.Ten pairs of carpal, metacarpophalangeal and metatarso-phalangeal joints in 16 horses were included in the study.Four horses were geldings and 12 were mares.All horses were Thoroughbreds aged 3-15 years (mean 9.9 years) showing no clinical signs of lameness in the joints radiographed and weighing 446-618 kg (mean 499 kg).For the trial, lateromedial radiographic views were made of each joint in the following order: weight-bearing; passively flexed 45E, maximally flexed (stress flexed), passively flexed 45E and a weightbearing view were made followed by a dorsopalmar view in the weight-bearing position.If any gas was noted in the joints on these radiographs, an additional passively flexed 45E view was made approximately 5 min after the maximally flexed view.
Part 2 of the study evaluated 20 anaesthetised horses suspended from an overhead crane by means of leather hobbles attached to their phalanges.Seventeen Thoroughbreds, 2 Hanovarians, 1 Arab and 1 Friesian with weights ranging from 388-550 kg (average 455 kg) and ages from 2-14 years (average 5 years) were used.Radiographs were made in the induction room before the horses were moved into the theatre for surgical procedures not related to the radiographed limbs.Lateromedial views were made of either the left or right carpal, metacarpophalangeal and metatarsophalangeal joints.The 60 joints were chosen at random and were not evaluated clinically or radiographically before the procedure.All radiographs were made within 15 min of suspending the horse.
Part 3 of the study evaluated clinically and radiographically pathological joints.Radiographic pathology was diagnosed from the abovementioned standard views of the joint.This was followed by an additional lateromedial maximally flexed radiograph of the same joint.Twentyfour carpi from 20 horses and 15 metacarpophalangeal and 9 metatarsophalangeal joints from a different group of 20 horses were evaluated.Included in this study were 2 metatarsophalangeal joints and 1 carpus without radiologically-detectable pathology but with clinical lameness emanating from them.This was verified by intra-articular anaesthesia performed not less than 24 hours before making the maximally flexed radiographs.
All radiographs were made using non-grid exposures, Trimax T6 screens (3M, Elandsfontein, South Africa) and a source-to-image distance of 100 cm.Exposure factors of 90 kV 1 mAs were used for the metacarpo-and metatarsophalangeal joints, and 93 kV 1 mAs for the carpal joints to provide long-scale contrast for optimal soft-tissue and gas visibility.Standard automated radiographic development techniques were employed.
Radiographs were evaluated by all authors independently.Where opinions differed, consensus was obtained.Size, anatomical location and persistence of the gas was evaluated.When doubt existed whether a radiolucency was caused by superimposition and summation of bones or caused by vacuum phenomenon, it was interpreted as being caused by summation.

RESULTS
Gas was detected in 1 carpus, 1 metacarpophalangeal joint and 3 metatarsophalangeal joints in Part 1 of the study.
No gas was detected in any of the joints in Part 2 of the study.Carpal joints did not appear to be widened by the weight of the suspended horse whereas metacarpo-(metatarso) phalangeal joints widened up to 2 mm.It was difficult to obtain ideally positioned lateromedial views owing to the elevated position of the joints.Minor degenerative changes were present in some joints.
In Part 3 of the study, no gas was detected in any of the carpi radiographed.Gas was present in 1 metacarpophalangeal joint that showed no radiological pathology but had lameness localised to the joint.When joints that initially showed vacuum phenomena (Parts 1 and 3) were radiographed in a non-stressed flexed position 5 min later, no gas was detected.No additional views were made to see if gas was persistent, as no gas was present in Part 1 of the study when passively-flexed views were made 5 min after the maximally-flexed views were made.
In the carpus, gas accumulated in the intercarpal joint distal to the intermediate and radial carpal bones.Small periarticular osteophytes were present on the dorso-distal radial and intermediate carpal bones (Fig. 1).In the metacarpophalangeal joint a rectangular radiolucency was present dorsal to the articular surface of the sesamoids (Fig. 2).In the metatarsophalangeal joints, gas typically localised distal to the plantar articular surface of the condyles of the 3rd metatarsus (Fig. 3).

DISCUSSION
Analysis of gas aspirated from human lumbar discs with vacuum phenomenon consists of 92 % nitrogen combined with oxygen, carbon dioxide and traces of other gases 2 .The vacuum phenomenon has been explained by anatomical and physiological factors 2,12,13,15,16 .When opposing articular surfaces are distracted, a space is created that must be filled.If there is insufficient fluid to fill the expanded space, gas in the surrounding extracellular fluid escapes from solution and occupies the volume created by distraction, thereby producing the vacuum phenomenon, resulting in a physiological pneumoarthrogram.When the joint returns to opposition the space is obliterated and the gas is resorbed.If the joint is kept distracted, the gas disappears after a few minutes owing to the accumulation of fluid in the joint 3 .This may indicate that gas in solution diffuses more readily than fluid, which then later fills the available space.In the presence of synovial  effusion, a vacuum phenomenon will usually not occur with distraction of the articular surfaces 13,[15][16][17]19 . Thi, however, is not always the case, as a vacuum phenomenon has been demonstrated in a human knee with an acutely injured anterior cruciate ligament accompanied by the presence of effusion and haemarthrosis 3 .In joints with irregular articular surfaces the vacuum phenomenon has been shown to occur spontaneously in fissures and clefts of vertebral articulations 5,8 without traction.Movement evidently increases the space in these fissures, thereby attracting gas from the surrounding tissues 12,13 .
In humans the incidence of the spinal vacuum phenomenon increases with age 8 .In this trial, owing to insufficient numbers, the effect of age and sex on the incidence of the vacuum phenomenon could not be established.Traction-induced vacuum seen in hyper-extended shoulders and hips in paediatric radiographs is dependent on muscular development, muscle tone and laxity of the joint capsule and ligaments 9 .It is limited in advanced osteoarthritis by capsular thickening and muscular spasm 17 .
In humans, distraction with a resultant pneumoarthrogram is used as a means of detecting abnormalities of the articular cartilage.Absence of the vacuum phenomenon in distracted hips indicates the presence of synovial effusion 9,10 .Traction, rotation or abduction can create the phenomenon as apposing articular surfaces are distracted.Using an overhead crane in anaesthetised horses it was not possible to produce the vacuum phenomenon in joints under investigation.This difference may be due, in part, to our inability to cause sufficient distraction in equine joints and the time lapse between initiation of traction and making the radiograph.
In humans the vacuum phenomenon is a reliable indicator of intervertebral osteochondrosis, intervertebral ischaemic necrosis of bone and, rarely, spinal infection 1 .In the scapulo-humeral joint of dogs, the phenomenon is found with osteochondrosis 16 associated with a cartilage flap and a lack of joint effusion.In this trial, the presence of radiographic pathology in carpal, metacarpophalangeal, and metatarsophalangeal joints, was not associated with an increased presence of the vacuum phenomenon in stressflexed radiographs.Radiographic pathology present in the joints included osteochondral fragmentation and degenerative joint disease.The lack of vacuum phenomenon may be due to the presence of joint effusion 17 preventing space formation, or a compliant joint capsule 15 or the fact that stress flexion was resisted by most of the horses due to pain.Pain and resistance from the horse would result in difficulty in obtaining sufficiently flexed views and also delayed the time when the exposure was made, allowing time for any gas to disappear.Capsular thickening 15 due to osteoarthritis would also limit the degree of joint distraction.
In humans the absence of the vacuum phenomenon when manual traction is placed on the metacarpophalangeal joints is a reliable indicator of joint effusion 9 .This does not appear to be the case in horses.
Stress flexion of a horse's metatarsophalangeal 15 , metacarpophalangeal or carpal joints may induce the vacuum phenomenon which could lead to an incorrect interpretation and result in misdiagnosis of osteochondral defects or fractures.Radiolucencies attributable to gas may also be seen in radiographs of joints after arthrocentesis, surgery, sepsis, intra-synovial anaesthesia 6 or trauma.Table 1: Joints demonstrating vacuum phenomenon.Summation, i.e. radiological opacities created by the partial superimposition of more than one bony structure, may also result in relative radiolucencies, (negative Mach line) adjacent to it, mimicking a vacuum phenomenon 11 (Fig. 2).These apparent radiolucencies can be distinguished from the vacuum phenomenon by being less radiolucent and their borders continuing beyond the edges of adjacent bones.The presence of radiolucencies in radiographs of equine joints should always be interpreted with care and due consideration to the clinical findings.

Fig. 2 :
Fig. 2: Metacarpophalangeal joint.A rectangular radiolucency with multiple small oval/circular superimposed radiolucencies is present dorsal to the articular surface of the proximal sesamoid bones (large arrow).Note the apparent radiolucency (small arrow) caused by superimposition of bones.