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Maarten M. Hoogbergen - Bone mineral density patterns representing the loading

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GENERAL DISCUSSION

BONE MINERAL DENSITY PATTERNS REPRESENTING THE LOADING HISTORY OF THE WRIST JOINT

Force transmission through the wrist joint has baffled many generations of physicians. Many of the research done on this subject consisted of in vitro work on cadaver wrists bringing a variety of measuring devices into the wrist joint spaces1, 3, 4, 16, 20, 25, 26, 31, 41, 45-47, 50, 52, 53, 55, 56, 59, 60. The results of this research on force transmission through a normal carpal joint showed a pattern in which the radioscaphoid joint and the radiolunate joint are considered the predominant sites of force transmission (approximately 60% through the radioscaphoid and 40% through the radiolunate joint3, 25, 50, 80% to 85% through the radiocarpal joint and 15% to 20% through the distal ulna1, approximately 50% through the radioscaphoid and 50% through the radiolunate joint26). Depending on the ulnar variance, the distal ulna and TFCC play a part in the force transmission as well1, 4, 31, 39, 60.

These in vitro methods, however, have considerable limitations. The disturbance of the delicate local anatomy by bringing measuring devices into the joint spaces introduces the element of material bias in the results. Only momentary forces applied to the joint surfaces can be measured. There is no possibility of long-term follow-up. Besides, no assessment of in vivo force transmission is possible.

Although these in vitro methods showed a certain force-transmission pattern through the wrist, our objective was to measure in vivo long-term force transmission patterns through the wrist joint. In order to do so we used a method that takes into consideration the fact that bone adapts to changes in forces, acting upon its surface.

An increase in force will give rise to an increase in bone mineral density (BMD) likewise a decrease in force will give rise to a decrease in BMD. By measuring these changes in BMD, force transmission patterns can be quantified in vivo 6-9, 11-15, 17-19, 22, 29, 30, 33-38, 62. The results of these in vivo studies are in accordance with the in vitro studies.

To begin with, the feasibility of the Dual-Energy-X-ray-Absorptiometry scan (DEXA) was investigated by combining BMD patterns in normal and pathologically altered cadaver wrists, as well as in a patient with Kienböck’s disease. BMD pattern alterations could be observed. A shift of BMD was seen towards the scaphoid fossa in wrists with distal radius or proximal carpal row pathology, indicating a shift in force transmission towards this fossa.

Although this shift could be seen in the two-dimensional DEXA-scan, no mapping of the joint surface could be done. The imitation of the DEXA-scan is that it only shows the occurrence of BMD pattern changes in a two dimensional plane. In order to predict force transmission changes in the joint itself, three-dimensional BMD mapping of the entire joint surface is necessary. To obtain this information a Computer Tomography Absorptiometry (CT-A) scan was performed After validation of the CT-A scanning in cadaver wrists the BMD patterns of fifteen patients with distal radius, distal ulna or proximal carpal row pathology were compared.

This method provided us with information regarding in vivo force-transmission patterns in normal and pathologically altered wrists. Force-transmission patterns in the preoperative situation can be used to help to define the indication for operative intervention. Comparing the preoperative BMD patterns with the postoperative situation one can assess the result of an operation in terms of force transmission pattern normalization. At the same time this method can also be used to measure changes in force-transmission patterns over longer periods of time allowing longterm assessment of operative results.

A normal wrist shows a BMD pattern in which a bone density maximum is seen in the scaphoid fossa as well as in the lunate fossa, which accounts for approximately 40 % of the entire joint surface. On the outer site of the BMD centers, an evenly distributed BMD on the surface of the distal radius, distal ulna and proximal carpal row is seen. This is in accordance with the report by Viegas49 in which it was stated that even with the highest loads applied the contact areas remain small, no more than 40 % of the total joint surface. Small areas of increased BMD could be seen at the radiovolar aspect of the scaphoid fossa and at the ulnar styloid process. These increases in BMD are probably due to the insertion of the radio-scapho-capitate ligament (RSC) and the volar and dorsal radioulnar ligaments respectively.

This pattern is seen in all normal wrists and is in accordance with data obtained by in vitro1, 3, 4, 16, 20, 25, 26, 31, 41, 45-47, 50, 52, 53, 55, 56, 59, 60 and in vivo studies11-15, 18, 22, 33-38.

In patients with pathology of the proximal carpal row or the distal radius an abnormal intracarpal kinematic pattern is to be expected. An abnormal intracarpal  kinematic pattern will give rise to an abnormal force-transmission pattern leading to the development of degenerative osteoarthritic changes10.

Three of the patients who underwent a proximal row carpectomy (PRC) suffered from a scapholunate dissociation (SL) and two suffered from a morbus Kienböck. In patients suffering from a SL-dissociation a consistent and specific BMD pattern could be observed preoperatively.

An increase in BMD can be seen at the styloid process and at the dorsal rim of the scaphoid fossa. This indicates an increase in force transmission through these areas and is in accordance with the malposition of the scaphoid relative to its fossa. These sites of increased BMD coincide with the sites where osteoarthritis evelops in the wrist23, 51, 58. The results obtained by CT-A are in accordance with the in vitro studies done by Viegas51, 55, 56 and Blevens3.

An increase in BMD can also be noticed at the ulnovolar aspect of the lunate fossa. This indicates an increase in force transmission in that area which can be attributed to the malposition of the lunate relative to its fossa. The lunate is rotated dorsally and translated ulnarly by the uncoupling of the lunate and scaphoid. The wedge shape of the lunate predicts its dorsal rotation movements when the scapholunate ligament (SL) is ruptured24. Due to the space between the scaphoid and lunate in a SL dissociation the capitate is able to move proximally.

Although the lunate fossa is not prone to develop osteoarthritis, Jeffries23 noticed that if osteoarthritis occurs it is usually on the ulnar side of the lunate fossa and at the ulnar side of the lunate itself. This resembles the force transmission patterns found in our studies. Watson58 57, however, stated that the lunate fossa is hardly ever affected by osteoarthritis.

Only two of our patients who underwent a PRC had a morbus Kienböck, so no specific BMD patterns of this subgroup could be established preoperatively.

Regarding the BMD patterns after a PRC, consistent and specific BMD patterns could be observed. After a PRC the scaphoid fossa is unloaded. The lunate fossa becomes the predominant site of force transmission with a BMD maximum in its center. Although ulnar translocation of the lunate is to be expected after a PRC due to the more obtuse angle of the RSC-ligament, a BMD maximum is seen in the center of the lunate fossa. This indicates that after a PRC the center of forcetransmission is, unexpectedly, in the center of the lunate fossa.

A rotatory unstable situation is created after a PRC, due to a difference in curvature of the lunate fossa and the capitate, a relative increase in length of the RSC-ligament due to a more obtuse angle and the removal of the entire proximal row. However, long-term follow-up after a PRC revealed that heavy manual labour can be performed by patients who underwent a PRC21, 43. So some stability of the wrist must exist after a PRC without peak moments of force-transmission.

The pronation stability is provided by the RSC-ligament and the supination stability by the extensor carpi ulnaris44. With increased load of the extrinsic muscles, stability is increased.

The shape of the lunate fossa is that of a socket with a certain diameter. It is possible that the capitate with a smaller diameter (2/3 of that of the lunate fossa21) is drawn into the deepest point of the socket by the action of the extrinsic muscles.

No high peaks in BMD were seen in the lunate fossa indicating no high centers of force transmission. This is probably due to the type of motion of the capitate relative to the lunate fossa. This motion is a translation and rotation movement 21 so the force is transmitted through a greater part of the curvature of the lunate fossa than was to be expected if only the curvature differences are taken into account.

Four of the patients who underwent a lunate-capitate-triquetrum-hamate arthrodesis (LCTH) suffered from a scapholunate  dissociation (SL) and one suffered from a large cyst in the scaphoid. Regarding the BMD patterns after a LCTH, consistent and specific BMD patterns could be observed. After an LCTH arthrodesis the scaphoid fossa is unloaded. The lunate becomes the predominant site of force transmission with a BMD maximum in its center. Although the scaphoid is excised,

no increase in BMD in the lunate fossa can be noted nor can an increase in BMD be seen in the distal ulna. The force, therefore, must consequently be equally distributed over the lunate fossa and the distal ulna without creating peak forces. Although the scaphoid fossa seems to play a major part in force transmission through the carpal joint2, 3, 13, 18, 25, 27, 28, 33, 34, 41, 42, 48, 50-52, 55, 56, excision of the scaphoid and fusion of the lunate, capitate, triquetrum and hamate does not seem to increase the force transmission in the lunate fossa. After an LCTH-arthrodesis the BMD patterns in the lunate fossa resemble the BMD patterns after a PRC. An explanation for this may be the decreased use of the hand operated on due to a decrease mobility, which results after this type of surgery. Another reason could be that the follow-up is too short (one year) although Cowin5 states that the remineralisation process should be completed within several months.

The position of the lunate is restored in relation to its fossa by performing an LCTHarthrodesis. No BMD increase at the dorsal edge of the lunate fossa was seen, which reveals that no riding of the LCTH bone block at the dorsal rim of the lunate fossa occurred. Radiolunate osteoarthritis is regarded a rare occurrence after an LCTHarthrodesis32.

This is in accordance with the fact the LCTH-arthrodesis can restore the normal relationship between the lunate and lunate fossa in terms of force transmission patterns.

Five patients with symptomatic distal radio-ulnar joint instability underwent a Sauvé-Kapandji procedure. In four out of these five, an ulnar plus variance was seen on the x-rays, preoperatively. Besides the clinical DRUJ instability, their concomitant wrist pathology was so divers that no characteristic patterns regarding the BMD patterns of the radiocarpal joint could be made. This would have been of interest, because Viegas54 suggested that in cases of DRUJ instability in combination with interosseus membrane rupture, a possible increase in force, transmitted through the radiocarpal joint, could be observed. The BMD of the distal ulna however was increased in all patients with an ulnar plus variance, suggesting an increase in force transmission at that site. An increase in force transmission through the distal ulna, in an ulnar plus situation, is in accordance with in vitro studies conducted by Werner59, 60, Palmer39 and Schuurman40. As for the BMD patterns after a Sauvé-Kapandji procedure, some observations regarding the BMD patterns of the distal radius and ulna could be made.

By performing a Sauvé-Kapandji procedure with levelling of the radius in patients with an ulnar plus deformity, an unloading effect can be observed at the level of the distal ulna. This unloading effect is described by Werner59, 60 and Wnorowski61 in vitro. A decrease in BMD in the distal ulna was noted, although no increase in BMD of the radiocarpal joint was observed. This suggests that no increase in force transmitted through the radiocarpal joint occurs after a Sauvé-Kapandji procedure.

With this operative procedure, DRUJ instability can be treated with the possibility to change an ulna plus variance into an ulna neutral one without jeopardizing the radiocarpal joint, in terms of force transmission pattern changes.

BMD pattern evaluation, by using CT-A, for long-term force-transmission assessment in the wrist joint under normal and pathological conditions as well as after surgical interventions seems an adequate tool. This method can be used in vivo without disturbing the joint anatomy and can be repeatedly used to obtain real longterm effects in terms of force transmission through the wrist joint. It is therefore a valuable tool to evaluate operative procedures at the wrist joint, and assess whether these are beneficial or will create osteoarthritis at another location in the course of time.

In the future, when more powerful CT scans and computers will become available, our method will more easily assess the data obtained from patients suffering from wrist pathology. The method described will then be a valuable tool for clinicians dealing with wrist pathology to make decisions regarding operative interventions, to follow-up patients after wrist surgery and to continue research on the relation between carpal kinematics and force transmission.
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