Kinematics of the midcarpal and radiocarpal joint in flexion and extension: an in vitro study

PURPOSE: To apply carpal kinematic analysis using noninvasive medical imaging to investigate the midcarpal and radiocarpal contributions to wrist flexion and extension in a quasidynamic in vitro model. METHODS: Eight fresh-frozen cadaver wrists were scanned with computed tomography in neutral, full flexion, and full extension. Body-mass-based local coordinate systems were used to track motion of the capitate, lunate, and scaphoid with the radius as a fixed reference. Helical axis motion parameters and Euler angles were calculated for flexion and extension. RESULTS: Minimal out-of-plane carpal motion was noted with the exception of small amounts of ulnar deviation and supination in flexion. Overall wrist flexion was 68 degrees +/- 12 degrees and extension was 50 degrees +/- 12 degrees. In flexion, 75% of wrist motion occurred at the radioscaphoid joint, and 50% occurred at the radiolunate joint. In extension, 92% of wrist motion occurred at the radioscaphoid joint, and 52% occurred at the radiolunate joint. Midcarpal flexion/extension between the capitate and scaphoid was 0 degrees +/- 5 degrees in extension and 10 degrees +/- 13 degrees in flexion. Midcarpal flexion/extension between the capitate and lunate was larger, with 15 degrees +/- 11 degrees in extension and 22 degrees +/- 19 degrees in flexion. CONCLUSIONS: The capitate and scaphoid tend to move together. This results in greater flexion/extension for the scaphoid than the lunate at the radiocarpal joint. The lunate has greater midcarpal motion between it and the capitate than the scaphoid does with the capitate. The engagement between the scaphoid and capitate is particularly evident during wrist extension. Out-of-plane motion was primarily ulnar deviation at the radiocarpal joint during flexion. These results are clinically useful in understanding the consequences of isolated fusions in the treatment of wrist instability.     

Changes in radiocarpal loading characteristics after proximal row carpectomy

PURPOSE: To document the changes in wrist loading that occur after proximal row carpectomy in a cadaver model. METHODS: The normal radioulnar carpal pressure distributions of 7 cadaver wrists were measured using super-low-pressure-sensitive film. Proximal row carpectomies were performed and the loading characteristics re-evaluated. RESULTS: In the lunate fossa the contact area increased 37%, the average contact pressure increased 57%, and the location of the contact moved radially 5.5 mm. With wrist motion between 40 degrees of extension and 20 degrees of flexion the volar/dorsal excursion of the lunate fossa contact point increased by 108%. CONCLUSIONS: Significant changes in radiocarpal loading occur after proximal row carpectomy. The increased radiocarpal excursion with wrist motion may explain the low incidence of radiocapitate arthritis in patients who have had proximal row carpectomy.     

Kienbock's disease: diagnosis and treatment

Kienbock's disease, or osteonecrosis of the lunate, can lead to chronic, debilitating wrist pain. Etiologic factors include vascular and skeletal variations combined with trauma or repetitive loading. In stage I Kienbock's disease, plain radiographs appear normal, and bone scintigraphy or magnetic resonance imaging is required for diagnosis. Initial treatment is nonoperative. In stage II, sclerosis of the lunate, compression fracture, and/or early collapse of the radial border of the lunate may appear. In stage IIIA, there is more severe lunate collapse. Because the remainder of the carpus is still uninvolved, treatment in stages II and IIIA involves attempts at revascularization of the lunate-either directly (with vascularized bone grafting) or indirectly (by unloading the lunate). Radial shortening in wrists with negative ulnar variance and capitate shortening or radial-wedge osteotomy in wrists with neutral or positive ulnar variance can be performed alone or with vascularized bone grafting. In stage IIIB, palmar rotation of the scaphoid and proximal migration of the capitate occur, and treatment addresses the carpal collapse. Surgical options include scaphotrapeziotrapezoid or scaphocapitate arthrodesis to correct scaphoid hyperflexion. In stage IV, degenerative changes are present at the midcarpal joint, the radiocarpal joint, or both. Treatment options include proximal-row carpectomy and wrist arthrodesis.     

Proximal row carpectomy

Excision of the proximal carpal row has proven over the past 60 years to be an effective technique for certain disorders of the wrist, including degenerative sequelae of scapholunate ligament dissociation and scaphoid nonunions, Kienböck disease, Preisser disease, and other fracture-dislocations of the wrist. The durability of this procedure may be due to remodeling of the capitate head to the lunate fossa. Poor results have been noted in patients with rheumatoid arthritis and arthrogrypotic wrist deformities. Preservation of relatively normal cartilage of the capitate head as well as the lunate fossa is critical for success of this procedure. Average outcomes of this procedure include a wrist extension-flexion arc of 75° and grip strength of 60% of the uninvolved wrist. Revision to total wrist arthrodesis is required in 10% of patients. Approximately 90% have pain relief and can return to moderate use activity. Copyright © 2001 by the American Society for Surgery of the Hand     

Scaphoid nonunion and distal fragment resection: analysis with three-dimensional rigid body spring model

Background  Distal fragment resection is one of the salvage procedures for scaphoid nonunion with osteoarthritis. Despite being reported as a simple procedure with favorable midterm outcomes, further arthritic changes remain a concern in the long term. Scaphoid waist fracture is classified into volar or dorsal types according to the displacement pattern, but the indications for distal fragment resection have never been discussed for these fracture types. Method  We reconstructed a normal wrist model from computed tomography images and performed theoretical analysis utilizing a three-dimensional rigid body spring model. Two types of scaphoid fracture nonunion followed by distal fragment resection were simulated. Results  With volar-type nonunion, the force transmission ratio of the radiolunate joint increased, and the pressure concentration was observed in the dorsal part of the scaphoid fossa and volar part of the lunate fossa of the radius; no deterioration was seen in the midcarpal joint. In the distal fragment resection simulation for volar-type nonunion, pressure concentrations of the radiocarpal joint resolved. With dorsal-type nonunion, force transmission ratio in the radiocarpal joint resembled that of the normal joint model. Pressure concentrations were observed in the dorsoulnar part of the scaphoid fossa and radial styloid. The pressure concentration in the dorsoulnar part of the scaphoid fossa disappeared in the resection model, whereas the concentration in the radial styloid remained. In the midcarpal joint, pressure was concentrated around the capitate head in the nonunion model and became aggravated in the resection model. Conclusions  With volar-type scaphoid nonunion, distal fragment resection seems to represent a reasonable treatment option. With dorsal-type nonunion, however, pressure concentration around the capitate head was aggravated with the simulated distal fragment resection, indicating a potential risk of worsening any preexisting lunocapitate arthritis.     

Intercarpal and radiocarpal resection arthroplasty and arthrodesis

Indications for intercarpal and radiocarpal resectionarthroplasty and fusions are osteoarthritis, KIEHNBOCK'S disease, rheumatoid arthritis and several posttraumatic disorders of the wrist joint. The resection of carpal bones leads to severe instability patterns of the wrist. In conclusion we recommend resection-arthroplasty just for treatment of the thumb carpo-metacarpal osteoarthritis. Implant resection arthroplasty of the lunate and scaphoid or total wrist implants are still causing multiple problems regarding heavy load. Therefore this implants should be confined to rheumatoid patients. Of the limited carpal arthrodeses the scaphotrapezium-trapezoid arthrodeses is the most frequent performed procedure. It can be indicated for STT-osteoarthritis, KIEHNBOCK's disease, scapho-lunate instability and scaphoid pseudarthrosis if other surgical procedures had failed presuming there are no signs of arthrosis in the radiocarpal joint. Persisting pain especially in heavy work is quite frequent after limited arthrodesis but can be greatly relieved by simultaneous wrist denervation. In advanced cases of osteoarthritis total wrist arthrodesis is still the best choice for the patient.     

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.     

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.     

Functional results after proximal row carpectomy to salvage a wrist

Proximal row carpectomy is a movement-preserving procedure in the treatment of arthrosis of the wrist. We have retrospectively assessed the objective and subjective functional results after proximal row carpectomy. Assessment of outcome included measurement of range of movement (ROM), grip strength and self-assessment of pain relief with a visual analogue scale (VAS) and the Disabilities of arm, shoulder, and hand (DASH) questionnaire. Results were graded using the Mayo and Krimmer wrist scores. Fourty-five patients (mean age 48 (30–67) years) were evaluated with a follow-up of 32 (8–115) months. Underlying conditions included: degenerative arthritis secondary to scapholunate advanced collapse deformity, or chronic scaphoid non-union (n=35), Kienböck disease stage III (n=4), chronic perilunate dislocation and fracture-dislocation (n=4), avascular necrosis of the scaphoid (n=1), and severe radiocarpal arthrosis secondary to distal radial fracture (n=1). Active ROM for wrist extension and flexion was 70° and mean radial and ulnar deviation 30.8°. Grip strength was 51% of the unaffected side. The average DASH score was 26. The intensity of the pain, measured by VAS, was reduced by 44% after strenuous activities and by 71% at rest. The Mayo and Krimmer wrist scores were 55 and 62 points indicating good results; 32 patients returned to work and 25 patients to their former occupation. Our results show that proximal row carpectomy is a technically easy operation that preserves a satisfying ROM and pain relief, and is recommended when the head of the capitate and the lunate fossa are not affected by arthrosis.     

Proximal row carpectomy

Proximal row carpectomy is extremely useful as a wrist reconstructive technique for cases of degenerative joint arthritis of the radiocarpal joint cause by scapholunate advanced collapse, scapholunate advanced collapse, schaphoid nonunion advanced collapse, trans-scaphoid perilunate fracture dislocations, lunate dislocations, and Kienb?ck disease. It should be selected with caution for patients younger than 35 years old. The procedure can be performed with or without temporary internal fixation with with Kirschner wires, and adjunctive techniques of dorsal capsule interposition, proximal capitate excision, and radial styloidectomy can be used. The longevity of the operation is good, but the patient should be informed preoperatively that secondary procedures may be required. Based on historical series, these procedures have included addition have included of radial styloidectomy when this has not been performed at the index procedure, revision of the surgery with capitate debridement or conversion to total wrist arthodesis. Conversion of proximal row carpectomy to total wrist arthoplasty with implants can be contemplated in selected patient particularly as newer implants are designed. The technique the senior author has used on occasion has been to perform revision surgery on those patients who have chronic pain who might need further debridement of the radius in the radial styloid, the proximal capitate, or evaluation of the integrity of the interposition.     

Proximal row carpectomy: clinical evaluation

Proximal row carpectomy as a treatment of disorders of the radiocarpal joint remains controversial despite numerous reports documenting clinically successful outcomes. Criticism includes postoperative loss of grip strength, unsatisfactory range of motion, prolonged rehabilitation time, and the potential for progressive painful arthritis. Twenty-seven patients were studied to address these concerns. The average length of follow-up was 4 years. Postoperative pain relief was achieved in 26 patients, allowing 24 of the 27 patients to return to their previous activity status within an average of 4.5 months after surgery. In all cases, range of motion matched or surpassed preoperative values. Grip strength improved to an average of 80% of the contralateral side. A detailed radiographic analysis of the radii of curvature of the lunate fossa and the capitate showed that the radius of curvature of the capitate is approximately two thirds of the corresponding value of the lunate. Motion between the capitate and the radius is translational with a moving center of motion, which may dissipate load on the radius and explain the relative success of the procedure.     

四角融合术对桡腕关节内应力的影响

目的 比较四角融合前后桡腕关节内应力值、受力面积和平均压强的变化,研究四角融合术对桡腕关节内应力的影响.方法 8例新鲜正常腕关节标本通过夹具固定于支架上,将150 N负荷按比例加载于经过腕关节的各肌腱上,调节腕关节于不同位置,测量桡舟、桡月关节内的应力大小、受力面积和平均压强,测量结束后对标本行四角融合术,以同样方法进行测量,比较四角融合前后数据,分析四角融合对桡腕关节的影响.结果 正常时,桡舟关节内受力面积占其关节面的3%～43%,桡月关节为6%～30%;四角融合后,桡舟关节为0%～30%,桡月关节为7%～32%.正常时和四角融合后,桡腕关节内的应力值、受力面积和平均压强随腕关节位置的变化而呈现相似的变化趋势,但正常时应力柔和渐变,四角融合后明显剧烈而僵硬.结论 正常桡舟、桡月关节内有各自独立的应力承受区,其面积不等于实际的关节面积,根据腕关节的位置不同而有变化;四角融合后桡腕关节内的应力变化较正常时剧烈、僵硬,长期作用可能使腕关节内软骨发生退行性变化.     

近排腕骨切除术后桡腕关节的解剖学及生物力学变化

目的研究近排腕骨切除术后新的桡腕-桡头关节的解剖学特点及生物力学变化，增强临床对这一术式的理解。方法6只福尔马林固定的尸体腕标本，模拟近排腕骨切除术后观测头状骨近端关节面；纵性剖开头状骨及月骨，描拓桡月关节和桡头关节的匹配曲线；选用6只新鲜标本牙托粉包埋后，用压敏片在双轴液压材料测试系统上测试正常和术后桡腕关节的接触面积和压力变化。数据进行统计学处理。结果正常桡月关节匹配曲线良好，腕骨切除术后头状骨近端关节面有4个子关节面，和桡骨远端的桡月窝匹配性差。腕中立位下，正常的平均腕单位面积负荷为（22．9±4．3）N／cm^2。近排腕骨切除术后为（136．4±30．7）N／cm。（P〈0．05）。结论近排腕骨切除术后形成的新桡腕关节-桡头关节其解剖学和生物力学都明显异常。     

Die Entfernung der proximalen Handwurzelreihe

Objective

Resection of the proximal carpal row, termed proximal row carpectomy (PRC), is performed in order to treat pathologies of the proximal carpal row or radiocarpal joint between the scaphoid and scaphoid facet. It entails the articulation of the capitate and the lunate facet.

Indications

Lunate necrosis, carpal collapse, joint infection with concomitant intercarpal ligament lesions.

Contraindications

Severe cartilage lesions of the lunate facet and the capitate, wrist capsule laxity, rheumatoid arthritis, neuromuscular dysbalance of the wrist-covering soft tissue structures.

Surgical technique

Dorsal approach to the wrist, incision of the third and fourth extensor compartments, resection and coagulation of the dorsal interosseous nerve, usage of a ligament-sparing capsule incision, identification of the proximal carpal row and inspection of cartilage of the lunate facet and capitate, mobilization and excision of the lunate, scaphoid and triquetrum, articulation of lunate facet and capitate is controlled clinically and fluoroscopically, wound closure, application of plaster slabs.

Postoperative management

Immobilization of the wrist on plaster slabs for 2 weeks, removal of sutures after 14 days.

Results

PRC is a surgical procedure with few complications. Satisfactory range of motion and grip strength could be preserved without limiting function of the upper extremity. Postoperative osteoarthritis of capitate and lunate facet did not correlate with the good clinical outcome.

     

Proximal row carpectomy with capitate osteochondral autograft transplantation

Proximal row carpectomy (PRC) is an effective treatment option for degenerative or posttraumatic osteoarthritis of the wrist in conditions such as scapholunate advanced collapse, scaphoid nonunion advanced collapse, Kienbock disease, and chronic fracture dislocations of the carpus. PRC involves excision of the scaphoid, lunate, and triquetrum, and relies on the articulation of the remaining capitate from the distal row to articulate with the lunate fossa. PRC offers the potential advantage of greater range of motion, technical ease, and decreased immobilization, and eliminates specific complications found with other motion-preserving procedures such as nonunion, hardware irritation, and impingement. An established relative contraindication for PRC is the presence of advanced capitolunate arthritis. Many authors have offered modifications of the traditional PRC procedure to account for the presence of capitate arthritis. A PRC technique utilizing an osteochondral autograft, from the carpal bank of excised bones, for transfer to the capitate defect is described.     

Evolution of arthritis of the wrist

Degenerative arthritis of the wrist follows very specific patterns from onset to terminal severe bone and joint destruction. About 95% of them occur as periscaphoid area problems: SLAC (scapholunate advanced collapse pattern) wrist (55%), triscaphe arthritis (26%), and a combination of the two (14%). In SLAC wrist, the repeating sequence of degenerative change is based on and caused by articular alignment problems between the scaphoid and the radius. Changes then progress between the capitate and the lunate that are secondary to carpal collapse. In triscaphe arthritis, the degenerative change is limited to between the trapezium, trapezoid, and distal scaphoid. SLAC procedure (fusion of the capitate, lunate, hamate, and triquetrum along with silastic scaphoid implant) for SLAC wrists and triscaphe arthrodesis (fusion of the scaphoid, trapezium, and trapezoid) for triscaphe arthritis, are designed to make maximum use of undamaged structures and to maintain full-power, painless, mobile human wrists.     

The anatomy of the ligaments of the wrist and distal radioulnar joints

The ligaments of the wrist are responsible for guiding and constraining the complex motion of the carpal bones relative to the forearm bones, the metacarpals, and contiguous carpal bones. The majority of wrist ligaments are found within the joint capsule as organized thickenings composed of parallel collagen fascicles, small caliber nerves and blood vessels, and lined on their deep surfaces by synoviocytes. The palmar radiocarpal ligament complex is composed of the radioscaphocapitate, long radiolunate, radioscapholunate and short radiolunate ligaments. The ulnocarpal ligaments include the ulnolunate, ulnotriquetral and ulnocapitate ligaments. Dorsally, the radiocarpal joint is spanned by the dorsal radiocarpal ligament. Palmar ligaments connecting the proximal and distal carpal rows include the scaphotrapeziotrapezoid, scaphocapitate, triquetrocapitate and triquetrohamate ligaments. Within each row are interosseous ligaments connecting adjacent carpal bones, each divisible into dorsal and palmar components. There are unique regions within some of the ligaments, such as a zone of fibrocartilage in the proximal regions of the scapholunate and lunotriquetral interosseous ligaments, and strong deep regions connecting the trapezoid, capitate, and hamate. The distal radioulnar joint is connected by the triangular fibrocartilage complex, composed of a fibrocartilaginous disc and the palmar and dorsal radioulnar ligaments. The ulnocarpal ligaments attach to the palmar radioulnar ligament rather than directly to the ulna, allowing increased independence between wrist and forearm motion.     

Three-corner wrist fusion using memory staples

Scapholunate dissociation with advanced collapse (SLAC), scaphoid nonunion advanced collapse (SNAC), and lunotriquetral advanced collapse (LTAC) of the carpus are challenging problems. Various treatment options have been described. We describe a technique of 3-corner wrist fusion, using memory staples. The scaphoid and triquetrum are resected, and the capitate is fused to the lunate. Articular cartilage is removed from the capitolunate joint, and the bones are shaped to conforming surfaces. Bone graft from the resected triquetrum and scaphoid is used to increase fusion rate and a dynamic compressive fixation force is applied due to the unique properties of the memory staples. The main advantages of this procedure include the following: retained anatomical articulation between the lunate and the lunate fossa on the radius, improved ulnar deviation due to the resection of the triquetrum, and an excellent fusion rate between the lunate and capitate due to the dynamic fixation, the conforming surfaces, and the use of autologous bone graft.     

Vascularized capitate transposition for advanced Kienböck disease: application of 40 cases and their anatomy

Based on anatomic study, the vascularized capitate transposition to replace excised necrotic lunate was designed and applied in 40 cases of advanced Kienb?ck disease. It includes excision of the necrotic lunate and proximal shift of the vascularized capitate. The blood supply of the transposed capitate is provided by the dorsal branch of the anterior interosseous artery. Bone union occurred radiographically, and no postoperative capitate necrosis occurred in all cases after 6 weeks. Twenty-three cases were followed up for 1 year. No residual wrist pain existed in the range of motion, but limited residual wrist pain existed in labor work. The arc of motion ranged on average from 35 degrees of flexion to 45 degrees of extension. The grip power of the affected hand reached on average 70% compared with the contralateral. The authors conclude the vascularized capitate transposition is a reliable alternative for advanced Kienb?ck disease.     

Load transmission through the wrist in the extended position

PURPOSE: The wrist is subjected to extremely high compressive loads in the extended position, but pathoanatomy of this region remains unclear. The purpose of this study was to analyze force transmission in the maximum extended position to clarify the pathomechanics of wrist injury. METHODS: Two sets of computed tomography images of wrist joints were obtained for 7 normal subjects: one set in neutral position and the other set in maximum extension. A three-dimensional rigid body spring model was used to analyze stress distributions through the wrist joint. The wrist joint was constructed from computed tomography images. External force was applied to the 5 metacarpals in neutral position and to the palm in extended position. Force transmissions through the carpus and ligament tension in extended position were compared with those in neutral position, and force distributions were compared in each position. RESULTS: Force transmission ratio on the scaphoid fossa significantly increased from 52% in neutral to 62% in extension (p< .05), whereas the ratio through the lunate fossa decreased from 42% to 36%. In the midcarpal joint, force to the scaphoid significantly increased from 60% to 69% (p< .05). Force distributions of the radiocarpal joint in the extended position moved on the center of the lunate fossa and interfossal ridge of the scaphoid fossa. The dorsal ridge of the radial articular surface appeared as the new contact area. Tension in 3 palmar intrinsic ligaments and the flexor retinaculum greatly increased in the extended position. CONCLUSIONS: Force transmission in the extended position shifted radially, concentrating at the scaphoid. We could show how bending force causes scaphoid fracture and concentration of force on the radius surface might cause intra-articular fracture coinciding with the fracture pattern introduced by Melone. The palmar intrinsic ligaments appear key to maintaining the carpal arch in push-up position. Our theoretical analysis could well explain several patterns of wrist injuries.