An in vitro analysis of wrist motion: the effect of limited intercarpal arthrodesis and the contributions of the radiocarpal and midcarpal joints

Radiocarpal and intercarpal arthrodeses were simulated in 12 fresh cadaver wrists by means of external fixation. Range-of-motion measurements were made before and after simulated arthrodesis and used to calculate the contribution of the midcarpal and radiocarpal joints to wrist motion, as well as the residual wrist motion after limited intercarpal arthrodeses. Relative contributions to wrist motion were as follows: wrist flexion: radiocarpal (RC) joint 63%, midcarpal (MC) joint 36%; wrist extension: RC joint 53%, MC joint 46%. The wrist motion remaining after simulated arthrodeses was as follows: capitate-hamate: flexion (Flx) 98%, extension (Ext) 92%, ulnar deviation (UD) 96%, radial deviation (RD) 90%; scaphoid-lunate: Flx 97%, Ext 91%, UD 90%, RD 91%; scaphoid-trapezium-trapezoid: Flx 86%, Ext 88%, UD 67%, RD 69%; scaphoid-lunate-triquetrum: Flx 91%, Ext 82%, UD 86%, RD 70%; capitate-lunate: Flx 70%, Ext 59%, UD 89%, RD 79%; capitate-hamate-triquetrum: Flx 88%, Ext 79%, UD 88%, RD 81%; hamate-triquetrum: Flx 90%, Ext 85%, UD 89%, RD 94%; scaphoid-trapezium-trapezoid-capitate: Flx 85%, Ext 77%, UD 64%, RD 57%.     

Proximal row carpectomy in perilunate dislocations and lunatomalacia

Proximal row carpectomy was performed in 13 wrists with old unreduced perilunate dislocations and in 4 wrists with Kienb?ck's disease. One failure was converted to wrist arthrodesis, and 2 patients were lost to follow-up. The remaining 14 patients were reexamined after 11 (3-19) years. Average ranges of wrist motion were 37 degrees of dorsiflexion, 30 degrees of palmar flexion, 5 degrees of radial deviation, and 24 degrees of ulnar deviation. Grip strength averaged two thirds of the uninvolved hand. Pain relief was achieved in 12 patients, who all returned to their previous occupations. The results according to Cooney's criteria were 1 good, 8 fair, and 5 poor.     

不同内固定在舟大小多角骨融合术中稳定性的比较研究

目的 比较舟骨和大、小多角骨(scaphoid-trapezium-trapezoid,STT)融合器与克氏针在STT融合术中内固定强度的差异.方法 12侧新鲜冷冻尸体前臂标本,随机分为融合器组(使用STT融合器)和克氏针组(使用克氏针)2组,模拟进行STT融合术,术后以夹具固定于腕动力测试仪上模拟腕关节主要活动.活动前后均拍摄腕关节标准正侧位X线片,测量桡舟角、桡舟间距、舟骨长度及STT融合体稳定角.结果 融合器组:当腕关节运动范围增加到屈曲45°、背伸40°、尺偏30°、桡偏15°时,客观指标与初始状态相比较变化差异均无统计学意义(P＞0.05).克氏针组:当腕关节运动范围增加到屈曲35°、背伸30°、桡偏10°时,客观指标与初始状态相比较变化差异均有统计学意义(P＜0.05).尺偏30°运动后融合体稳定角与初始状态相比较差异有统计学意义(P＜0.05).结论 STT融合器在舟骨和大、小多角骨融合术中内固定强度大于传统内固定物中的克氏针.     

Arthrodesis of the arthritic wrist

Although arthroplasty has developed tremendously in the last few years, there are still indications for arthrodesis of the wrist, particularly in nonrheumatoid patients where fairly heavy loads are imposed on the joint. Fusion is also a salvage procedure for failures of other procedures. The authors recommend fusion in a neutral position for unilateral or bilateral cases. Neutral is defined as 0 degree on the lateral with about 10 degrees ulnar deviation. This gives an arc of motion from pronation and supination, which substitutes for palmar flexion and dorsiflexion. The important factors in securing arthrodesis are: internal fixation with intramedullary pins, bony contact, bone graft if necessary, compression, and obtaining or maintaining rotation of the forearm. In selected cases arthrodesis of the wrist produces a very good result.     

部分腕骨融合术或切除术对腕关节运动影响的实验研究

目的 研究临床常用的部分腕骨融合术或切除术对腕关节运动的影响程度。方法 将１２侧新鲜上肢处理后,固定于特制的测试架上,在腕背部打入２根或多根克氏针,作舟头骨、舟大小多角骨、舟月骨、月三解骨、桡月骨、ｆｏｕｒ ｃｏｒｎｅｒ、头月骨融合术、舟骨切除＋ｆｏｕｒ ｃｏｒｎｅｒ、舟骨切除＋头月融合术。观测腕关节正常运动活动范围,及作上述不同部分腕骨融合术后腕关节屈曲、伸腕、尺偏、桡偏度数。结果 桡舟月骨融合     

新型外卡式人工全腕关节假体的设计与活动范围研究

目的通过测量及收集桡骨远端关节面解剖学数据,设计新型外卡式全腕关节假体;初步评价新型外卡式全腕关节置换后腕关节活动功能。 方法根据在50例成人腕关节正侧位片(由解放军昆明总医院影像科提供)测得桡骨远端关节面掌倾角与尺偏角,结合已有文献数据及国外人工全腕关节设计经验,设计新型外卡式人工全腕关节系统,掌倾角和尺偏角分别设计为10°和7°。活动范围测量实验采用自身配对设计,以6具新鲜冰冻保存成人上肢标本,使用聚甲基丙烯酸甲酯根据实验操作台固定器形状进行包埋处理。将标本固定于三维运动实验台上,分别于腕关节近端、远端各固定4个红外线可识别的球形标记物,向上肢标本掌屈、背伸、尺偏、桡偏4个方向施加4 Nm的纯力矩,使腕关节相应做上述运动。记录0载荷和最大载荷时腕关节运动状态的图像并通过Cotex 3.0运动捕捉系统软件记录并计算出腕关节在4个方向上的活动范围。术前测量完成后,对6例标本使用新型外卡式人工全腕关节进行全腕关节置换,重复上述实验步骤,测量新型人工腕关节假体置换后腕关节活动度。比较6具标本置换前与置换后在掌屈、背伸、尺偏、桡偏4个方向上的活动度。应用配对t检验对前后数据做统计学分析。 结果所有标本在行全腕关节置换手术过程中术程顺利,术后关节活动流畅。相同载荷下,术后腕关节在背伸、尺偏、桡偏方向的活动度较术前无统计学差异(P>0.05),术前掌屈方向最大活动度为(72.3±4.8)°,术后掌屈方向最大活动度为(54.9±6.1)°,二者差异有统计学意义(t＝8.34,P<0.01)。 结论新型外卡式全腕关节的设计在活动范围方面基本满足了腕关节的生物力学要求,值得进一步探究其性能。     

Four-corner arthrodesis: influence of the position of the lunate on postoperative wrist motion: a cadaveric study

PURPOSE: Evaluate the influence of the position of the lunate on postoperative wrist motion in four-corner arthrodesis. METHODS: Six upper cadaveric limbs were evaluated, comparing the total arc of motion in each wrist after simulating four-corner arthrodesis. The lunate was fixed in 3 different positions: neutral (0 degrees ), extended (30 degrees ), and flexed (20 degrees ). Statistical analyses (ANOVA and Bonferroni tests) were carried out to establish the significance of differences in articular motion in these 3 positions. RESULTS: Significant statistical differences were observed in full wrist extension. No significant differences, however, were found in flexion-extension total arc of motion, radial deviation, or ulnar deviation. CONCLUSIONS: According to our results in this cadaveric model, the position of the lunate affects postoperative wrist flexion and extension after four-corner arthrodesis. The flexed lunate position increases postoperative wrist extension and restrains wrist flexion. Inversely, the extended lunate position improves articular flexion and limits extension. Total arc of motion of the fused wrist does not vary in the 3 lunate positions.     

Lunate-triquetral and midcarpal joint instability.

Instability of the ulnar side of carpus centers around the triquetrum, which is suspended by the ulnar triquetral ligaments and supported proximally by the TFCC. The triquetrum guides the lunate by an interosseous membrane and stout palmar ligaments that provide a relatively rigid connection between the two bones. Disruption of the LT ligament is frequently associated with pathology in the ulnar carpal area and may progress to triquetral instability, VISI, and finally, degenerative arthritic changes on the ulnar side of the carpus. The diagnosis of LT injuries is made by stress radiographs, arthrography, video-fluoroscopy, and arthroscopy. Treatment is initially nonoperative, but if symptoms persist, surgery is warranted. Arthroscopic debridement and pinning the LT joint, ligament repair or reconstruction, and intercarpal arthrodesis have all been reported as successful treatments. For the chronic problem confined to the LT joint, a limited intercarpal arthrodesis of the joint is the most predictable procedure for relieving pain without causing any significant restrictions in wrist motions. When there is a dissociation pattern in addition to LT instability, a more extensive intercarpal arthrodesis is required. Midcarpal instability occurs at the triquetral-hamate joint and is characterized by a dynamic subluxation of the joint. During ulnar deviation, the joint undergoes an exaggerated shift from volar flexion to dorsiflexion. Supportive care is generally successful; although in chronic cases, a midcarpal joint arthrodesis is often required.     

Corrective osteotomy for symptomatic increased ulnar tilt of the distal end of the radius

Twelve wrists in 10 patients with a mean age of 23.6 years were treated for symptomatic increased ulnar inclination of the joint surface with corrective osteotomy of the radius. Diagnoses included mild ulnar dysplasia, posttraumatic deformity, Madelung's disease, and multiple hereditary exostosis. All patients had radial-sided wrist pain and an ulnarly displaced arc of radioulnar deviation. Preoperative radiographs showed excessive ulnar inclination of the distal radius, ulnar carpal translation, adaptive carpal malalignment, and frequent distal radioulnar joint incongruency. The patients had decreased pain and improved wrist function at a mean of 5.1 years (range, 2-10 years) after surgery. Average radial deviation changed from 3 degrees to 16 degrees and ulnar deviation from 48 degrees to 29 degrees; flexion/extension and pronosupination remained unchanged. Realignment of the wrist was shown radiographically by a change of ulnar inclination of the radius from 33 degrees to 21 degrees, an increase in scaphoid height from 16.4 to 20.4 mm, and reversal of ulnar carpal translation as shown by an increase in lunate-covering ratio of 64% to 77%. Reduction of the ulnar inclination to normal values by corrective radial osteotomy restores a more physiologic range of motion, decreases symptomatic wrist pain, reverts adaptive carpal changes to normal, increases lunate coverage, and may prevent abnormal cartilage overload in the ulnar compartment of the wrist.     

Intercarpal arthrodesis for static and dynamic volar intercalated segment instability

Since 1982 seven patients with volar intercalary segment instability (VISI) have been operated on at the Massachusetts General Hospital. All had preoperative wrist pain and described a painful "clunk" with ulnar deviation. In each case there was palpable evidence of instability when the wrist was deviated ulnarly that produced a "buckling" sensation as the distal and proximal rows rotated with ulnar deviation. Arthrograms in six patients and a cineradiography in one patient confirmed that this buckling correlated with volar rotation of the lunate and triquetrum and dorsal rotation of the capitate and hamate. All the patients had some type of intercarpal arthrodesis including four capitate-lunate-triquetrum hamate (CLTH), one lunate-triquetrum (LT), one lunate-triquetrum-hamate (LTH), and one triquetrum-hamate (TH). Surgical findings included the position of the lunate that had rotated on the capitate so that it was tilting volarly and the major ligament instability was between the proximal and distal rows although ligament tears were also present between lunate and triquetrum. Arthrodesis of the proximal and distal rows provided relief of wrist pain in five of six patients. The one patient with the arthrodesis limited to the proximal row had a poor result. Of the five successful cases, the postoperative range of wrist motion was 81 degrees of extension/flexion arc (63% of the normal contralateral) and 35 degrees of radial and ulnar deviation arc (57% of the normal contralateral wrist). The grip strength postoperatively averaged 58 pounds (74% of the normal contralateral side).     

腕投掷运动时腕关节韧带长度变化的活体研究

目的 探讨腕关节在投掷运动过程中腕关节韧带长度的变化.方法 对6例志愿者腕关节进行CT扫描,获取腕关节在投掷运动过程中的5个位置,即桡偏20°背伸60°,桡偏10°背伸30°,中立位,尺偏20°掌屈30°,尺偏40°掌屈60°时各腕骨、尺桡骨远段的三维重建图像,在重建图像基础上利用Mimics软件测得在腕关节投掷运动过程中掌、背侧腕关节韧带的长度.结果 腕关节由中立位至桡偏20°背伸60°时桡舟头韧带、长桡月韧带、尺头韧带、尺三角韧带长度显著伸长,分别延长(3.4±0.5)、(2.0±0.2)、(2.6±0.5)、(2.1±0.4)mm,差异均有统计学意义(P<0.05);腕关节由中立位至尺偏400掌屈60°时背侧桡腕韧带、背侧骨间韧带止于小多角骨部分长度显著伸长,分别延长(1.7 ±0.2)、(3.8 ±0.4)mm,差异有统计学意义(P<0.05).尺月韧带、背侧骨问韧带止于舟骨部分在投掷运动过程中其长度均较中立位时旱增长趋势.结论 腕关节在桡背伸至尺掌屈运动过程中,桡舟头韧带、长桡月韧带、尺头韧带、尺三角韧带缩短,提示张力减低,背侧桡腕韧带、背侧骨间韧带止于小多角骨部分伸长,张力增大,尺月韧带、背侧骨间韧带止于舟骨部分于中立位时张力最小,其变化规律有助于指导临床腕关节韧带损伤的修复.     

Cineradiographic study of wrist motion

To make clear the kinematics of the wrist in flexion-extension and radial-ulnar deviation, a cineradiographic study was carried out on twenty normal hands. The intercarpal angles were measured on each picture of the cineradiogram. The relationship between the wrist angle and the intercarpal angles were calculated by a micro-computer giving a 5th degree regression curve. From the maximal dorsiflexion to the neutral position, the wrist moves in the radiocarpal joint more than in the midcarpal joint, while, from the neutral position to the maximal palmar flexion, the wrist moves more in the latter. From the neutral position to the maximal radial deviation, the wrist motion occurs in the midcarpal joint more than in the radiocarpal joint, while, from the neutral position to maximal ulnar deviation, the wrist motion occurs equally in the two joints. The movements of the carpal bones were regulated by the tension and the relaxation of the carpal ligaments.     

Kinematics of the midcarpal and radiocarpal joints in radioulnar deviation: an in vitro study

PURPOSE: Carpal kinematics have been studied widely yet remain difficult to understand fully. The noninvasive measurement of carpal kinematics through medical imaging has become popular. Studies have shown that with radial deviation the scaphoid and lunate flex whereas the capitate moves radiodorsally relative to the lunate. This study investigated the midcarpal and radiocarpal contributions to radial and ulnar deviation of the wrist. This was accomplished through noninvasive characterization of the scaphoid, lunate, and capitate using 3-dimensional medical imaging of the wrist in radial and ulnar deviation. METHODS: Eight fresh-frozen and thawed cadaveric wrists were used in an experimental set-up that positioned the wrist through spring-scale actuation of the 4 wrist flexor and extensor tendon groups. The wrists were scanned by computed tomography in neutral and full radial and ulnar deviation. Body mass-based local coordinate systems were used to track the motion of the capitate, lunate, and scaphoid with the radius as a fixed reference. Helical axis motion and Euler angles were calculated from neutral to radial and ulnar deviation for the capitate relative to the radius, lunate, and scaphoid and for the lunate and scaphoid relative to the radius. RESULTS: The capitate, scaphoid, and lunate moved in a characteristic manner relative to the radius and to one another. Radial and ulnar deviation occurred primarily in the midcarpal joint. Midcarpal motion accounted for 60% of radial deviation and 86% of ulnar deviation. In radial deviation the proximal row flexed and the capitate extended; the converse was true in ulnar deviation. CONCLUSIONS: Radioulnar deviation (in-plane motion) occurred mostly through the midcarpal joint, with a lesser contribution from the radiocarpal joint. The results of our study agree with previous investigations that found the scaphoid and lunate flex in radial deviation (out-of-plane motion) relative to the radius whereas the capitate extends (out-of-plane motion) relative to the scaphoid/lunate (with the converse occurring in ulnar deviation). Our study shows how these out-of-plane motions combine to produce in-plane wrist radioulnar deviation. The use of 3-dimensional visualization greatly aids in the understanding of these motions. The results of our study may be useful clinically in understanding the consequences of isolated midcarpal fusions in the treatment of wrist instability.     

Rheumatoid wrists treated with synovectomy of the extensor tendons and the wrist joint combined with a Darrach procedure.

Forty-three rheumatoid wrists in 43 patients with bilateral wrist involvement were treated with synovectomy of the extensor tendons and wrist joint combined with a Darrach procedure in the period from 1966 to 1986. Clinical and radiologic assessment of the wrists was carried out after an average follow-up period of 11 years, with comparison of the treated and the opposite untreated wrists. The authors confirmed what others have concluded regarding the operation: pain was generally decreased, forearm rotation increased, and wrist extension and palmar flexion changed little. Radiologically, carpal collapse and palmar carpal subluxation progressed nearly parallel to the opposite wrists, but ulnar carpal shift was much greater in the surgically treated wrists. Therefore it is suggested that some measure to prevent ulnar carpal shift, such as Clayton's tendon transfer or radiolunate arthrodesis, should be included in this operation.     

In vivo elongation of the palmar and dorsal scapholunate interosseous ligament

PURPOSE: To investigate the elongation of the palmar and dorsal subregions of the scapholunate interosseous ligament (SLIL) in healthy human subjects throughout a complete range of wrist motion. METHODS: The 3-dimensional in vivo kinematics of the scaphoid and lunate were determined in both wrists of 13 female and 13 male volunteers from computed tomography volume images. For each wrist the palmar and dorsal insertions of the SLIL were identified on reconstructed surface models of the scaphoid and lunate. The interbone distances between the palmar and dorsal sites were calculated for the neutral wrist position. Elongations were then calculated after applying the 3-dimensional kinematics to the scaphoid and lunate. A multiple linear regression model was used to determine if elongations varied significantly as a function of wrist flexion/extension and radioulnar deviation. RESULTS: From pure wrist extension to pure wrist flexion, the fibers of the SLIL at the palmar insertion site increased significantly, from 29% shortening to 27% elongation, and the dorsal insertion decreased from 26% to 4% shortening with respect to the fiber lengths in the neutral position. From pure radial deviation to pure ulnar deviation, the elongation of the palmar insertion significantly decreased from 9% elongation to 21% shortening. There was no notable change in dorsal elongation with wrist radioulnar deviation. The multiple linear regression model predicted that there would be minimal elongation of the palmar and dorsal fibers at the wrist position along the dart thrower's path from radial extension to ulnar flexion. CONCLUSIONS: In vivo elongation of the palmar and dorsal fibers of the SLIL varied with wrist position. The palmar fibers lengthened and the dorsal fibers shortened with wrist flexion and the opposite occurred with wrist extension. Scapholunate interosseous ligament elongation was minimal as the wrist was positioned along the dart thrower's path.     

Effect of radial shortening on muscle length and moment arms of the wrist flexors and extensors

The purpose of this study was to determine the effect of distal radial shortening on muscle length and moment arm of the wrist flexors and extensors. In eight cadaveric upper extremities, distal radius fractures were simulated by an ostectomy. The distal radius was progressively shortened by 2.5, 5.0, 7.5, and 10 mm. Changes in the resting length of the flexor carpi radialis and ulnaris, extensor carpi radialis longus and brevis, and extensor carpi ulnaris muscles were measured with rotary potentiometers at neutral position, flexion, extension, and radial and ulnar deviation of the wrists. The wrists were passively moved through flexion-extension and radioulnar deviation, and tendon excursions and wrist joint angulation were recorded simultaneously. Tendon moment arms were derived from tendon excursions and joint motion. The results showed that either muscle, length or moment arm of the principal wrist flexors and extensors was significantly affected by the radial shortening. Muscle length decreased significantly after radial shortening in all the wrist flexors and extensors except for the extensor carpi ulnaris. The moment arm of the extensor carpi ulnaris tendon decreased significantly during either wrist flexion-extension or radioulnar deviation. The extensor carpi radialis brevis and flexor carpi ulnaris tendons also showed a significant decrease in their moment arms during radioulnar deviation of the wrist. Radial shortening of only 2.5 mm caused statistically significant changes in muscle length and moment arm of the wrist flexors and extensors. Increasing the extent of radial shortening exaggerated the biomechanical changes in the wrist motors. These results validate the importance of normal radial length for wrist kinetics and, from a biomechanical perspective, support complete correction of radial shortening after distal radius fractures.     

Distraction resection arthroplasty of the wrist

Proximal row carpectomy should not be done if wrist degeneration includes cartilage destruction of the capitate or lunate fossa of the radius; yet total wrist arthroplasty has been disappointing for treatment of osteoarthritis. We have used a technique we call distraction resection wrist arthroplasty in such cases. We retrospectively reviewed distraction resection wrist arthroplasty in 14 wrists and compared them to nine patients who had PRC; average follow-up was 32 months. Three patients had spastic contractures and 20 had operation for painful osteoarthritis. Patient satisfaction was high but there were four failures requiring arthrodesis (1 PRC, 3 DRA). Average postoperative wrist motion was 41 degrees of extension, 38 degrees of flexion, 11 degrees of radial deviation, and 13 degrees of ulnar deviation. Static strength averaged almost two thirds of the uninvolved side and dynamic power more than half. Differences between the DRA and PRC patients were not statistically significant in single or aggregate analysis. We believe that distraction technique extends the indications for biologic arthroplasty to patients whose only prior option was wrist arthrodesis.     

The effects of wrist distraction on carpal kinematics

Changes in carpal kinematics under wrist distraction were studied in fresh cadaveric specimens. A magnetic tracking device measured kinematic motions of the scaphoid, lunate, and third metacarpal relative to the fixed radius in 3 planes of passive motion (coronal, sagittal, and "dart throwers") under progressive distraction loads. The change in percent contribution of the radiocarpal and midcarpal joints was calculated. Radiocarpal motion during extension was decreased as increasing traction was applied, but it increased with flexion. Motion of the scaphoid relative to the lunate was smaller in the oblique plane, resulting in less radiocarpal motion than in the sagittal plane. In the coronal plane, traction had little effect on radial deviation, but ulnar angulation of the scaphoid was greater with ulnar deviation of the wrist. These results suggest that different degrees of tension exist in the palmar and dorsal ligaments with the wrist under traction and during different planes of wrist motion. If wrist motion is desired during fixed traction, such as used clinically with external fixation, the dart-throwers motion (wrist extension with radial deviation and wrist flexion with ulnar deviation) appears to have the least impact on radiocarpal motion. If greater radiocarpal motion is desired, however, such as during postoperative mobilization, flexion-extension and radioulnar deviation will create more radiocarpal motion than the dart-thrower's motion.     

Long-term study of chronic scapho-lunate instability treated by scapho-trapezio-trapezoid arthrodesis

The effect of scapho-trapezio-trapezoid arthrodesis on wrist kinematics was studied in 25 patients with chronic static scapho-lunate instability and in 16 patients with dynamic instability, with follow-up ranging from 24 to 101 months (average, 56 months). Postoperative planar and cineradiographic examination in patients returning to heavy labor reveal an absence of carpal shift-influence of the scaphoid proximal pole on the lunate-triquetral unit in ulnar deviation. Scapholunate diastasis present before operation persists in ulnar deviation as the STT fusion mass, capitate, and hamate rotate with the hand into ulnar deviation; the lunate-triquetral unit is not physiologically "pulled" radially into the lunate fossa of the radius. Triquetro-hamate mechanics remain normal as active engagement along the helicoidal triquetro-hamate interface initiates proximal row dorsiflexion. Clinical postarthrodesis wrist motion is a combination of intercarpal and radiocarpal mechanics, with energy in the flexion and extension arc dissipated through the scapho-lunate interface. The paucity of arthritic changes after up to 101 months after surgery is based on the kinematic changes presented in these data.     

In vivo radiocarpal kinematics and the dart thrower's motion

BACKGROUND: Wrist motion is dependent on the complex articulations of the scaphoid and lunate at the radiocarpal joint. However, much of what is known about the radiocarpal joint is limited to the anatomically defined motions of flexion, extension, radial deviation, and ulnar deviation. The purpose of the present study was to determine the three-dimensional in vivo kinematics of the scaphoid and lunate throughout the entire range of wrist motion, with special focus on the dart thrower's wrist motion, from radial extension to ulnar flexion. METHODS: The three-dimensional kinematics of the capitate, scaphoid, and lunate were calculated from serial computed tomography scans of both wrists of fourteen healthy male subjects (average age, 25.6 years; range, twenty-two to thirty-four years) and fourteen healthy female subjects (average age, 23.6 years; range, twenty-one to twenty-eight years), which yielded data on a total of 504 distinct wrist positions. RESULTS: The scaphoid and lunate primarily flexed or extended in all directions of wrist motion, and their rotation varied linearly with the direction of wrist motion (R2= 0.90 and 0.82, respectively). Scaphoid and lunate motion was significantly less along the path of the dart thrower's motion than in any other direction of wrist motion (p < 0.01 for both carpal bones). The scaphoid and lunate translated radially (2 to 4 mm) when extended, but they did not translate appreciably when flexed. CONCLUSIONS: The dart thrower's path defined the transition between flexion and extension rotation of the scaphoid and lunate, and it identified wrist positions at which scaphoid and lunate motion approached zero. These findings indicate that this path of wrist motion confers a unique degree of radiocarpal stability and suggests that this direction, rather than the anatomical directions of wrist flexion-extension and radioulnar deviation, is the primary functional direction of the radiocarpal joint.