Theoretical stress analysis in wrist joint--neutral position and functional position

Management of Lunate Facet Escape after Distal Radius Fracture Volar Plating: Surgical Technique

Fractures involving the lunate facet of the distal radius (commonly called the volar ulnar corner or "critical corner") are often difficult to recognize radiographically. "Lunate facet escape" refers to the displacement of this fragment after attempted distal radius fracture closed or open reduction methods. Lunate facet escape can have severe consequences including carpal collapse with pain, instability, and arthritis. Challenges to lunate facet fixation include irregular cortical contour that makes plate fixation difficult, as well as deforming forces by inserting structures. The goal of the management of a distal radius fracture with lunate facet involvement is first identification, then adequate stabilization until osseous healing occurs. However, subsequent lunate facet escape can pose a challenging clinical and surgical dilemma. Here we discuss our preferred approach to treat lunate facet escape after distal radius fracture volar locking plating failure.

Volar Lunate Facet Fractures of the Distal Radius: Fracture Mapping Using 3D CT Scans

Background  Fractures of the distal radius involving the lunate facet at the volar articular surface are unstable injuries and are usually managed operatively. Management of these fractures is challenging as our understanding of the exact fracture characteristics and associated injuries to the carpus is poor. Purpose  This study aims to define the anatomy and associated injuries of lunate facet fractures using three-dimensional computed tomography (CT) scans and fracture mapping techniques. Methods  A consecutive series of CT wrists was analyzed to identify intra-articular fractures involving the lunate facet at the volar distal radius. Fractures were mapped onto standardized templates of the distal radius using previously described fracture mapping techniques. We also identified instabilities of the carpus including volar carpal translation, ulnar translocation, scapholunate diastasis, and distal radioulnar joint (DRUJ) instability. Results  We present 23 lunate facet fractures of the distal radius. The lunate facet fragment displaces in a volar and proximal direction and the lunate always articulates with the displaced fragment. The smaller fragments displace a greater amount, in a volar direction, with pronation. The fracture tends to occur between the origin of the short and long radiolunate ligaments. Conclusion  Lunate facet fractures are frequently comprised of osteoligamentous units of the distal radius involving the short and long radiolunate ligaments and the radioscaphocapitate ligament. Assessment and management of volar carpal subluxation, scapholunate instability, ulnar translocation, and DRUJ instability should be considered. Clinical relevance  Our mapping of these fractures contributes to our understanding of the anatomy and associated instabilities and will aid in surgical planning and decision making.

Assessment and Management of Acute Volar Rim Fractures

The volar rim of the distal radius is the only bony restraint to volar carpal subluxation. Higher loads across the volar rim require stable and rigid fixation to maintain reduction and allow healing while rehabilitation begins. Volar marginal fragments are not amenable to buttressing by fixed angle volar locking plates. Appropriate management of volar marginal fragments comprises two steps-recognition of their presence and rigid anatomical repair. The best opportunity for success in the presence of a volar marginal fragment is its adequate initial treatment. The purpose of this review is to reinforce the importance of a complete preoperative and intraoperative evaluation of distal radius fractures. Volar marginal fragments can easily be overlooked even following initial reduction and fixation. Understanding the relevant anatomy and loading parameters can facilitate intraoperative decisions on approach and fixation, which are integral to achieving optimal clinical outcomes.

Musculoskeletal Modeling of the Wrist via a Multi Body Simulation

In this study, three different musculoskeletal modeling approaches were compared to each other. The objective was to show the possibilities in the case of a simple mechanical model of the wrist, using a simple multi-body-simulation (MBS) model, and using a more complex and patient-specific adaptable wrist joint MBS model. Musculoskeletal modeling could be a useful alternative, which can be practiced as a non-invasive approach to investigate body motion and internal loads in a wide range of conditions. The goal of this study was the introduction of computer-based modelling of the physiological wrist with (MBS-) models focused on the muscle and joint forces acting on the wrist.

Anatomy, Biomechanics, and Loads of the Wrist Joint

The wrist is by far the most differentiated section of the musculoskeletal system. The spectrum of wrist injuries ranges from minor injuries to complex traumas with simultaneous loss of functions, resulting in enormous economic costs. A proper understanding of the anatomy and biomechanics is essential for effective treatment, whether conservative or surgical; this applies to the wrist no less than to other parts of the human body. Here; information on the wrist anatomy; kinematics; and biomechanical behavior is presented, commencing with a brief explanation of the structure of its hard and soft tissues. Eight carpal bones in combination with two forearm bones (radius and ulna) construct the wrist joint. The motion of the wrist joint is initiated by the muscles of the forearm, and strong and short ligaments ensure the stability of the wrist. All of these components are essential to bringing functions to the wrist joint because these structures allow wrist mobility and sustainability. In addition, the kinematics of the wrist joint is presented and different biomechanical model approaches. The therapeutic (surgical) restoration of the balance between the load–bearing capacity and the actual stress on a joint is the prerequisite for a lifelong and trouble-free function of a joint. Regarding the complex clinical problems, however, a valid biomechanical wrist joint model would be necessary as assistance, to improve the success of systematized therapies based on computer–aided model–based planning and intervention.

Lunate loads following different osteotomies used to treat Kienböck's disease: A 3D finite element analysis

BACKGROUND

One of most accepted principles for treating Kienböck's disease before wrist degeneration settles in is to decompress the lunate by an osteotomy. Several osteotomies have been proposed since 1935. However, they are based on biomechanical hypotheses that are sometimes conflicting: This study compares the decompression effect of radius transverse shortening, radius lateral closing and medial closing wedge osteotomies, capitate shortening - with and without hamate shortening - and a Camembert-type radius wedge osteotomy with and without ulnar head shortening according to Sennwald.

METHODS

We built a 3D wrist model using finite elements that included the metacarpal, carpal and forearm bones. All wrist ligaments and Triangular Fibrocartilage Complex were incorporated in the simulation. Load was applied on the metacarpals with the forearm bones fixed. We then applied the different osteotomies to the model.

FINDINGS

When load was applied to the wrist, the osteotomies that best unloaded the lunate were the capitate shortening osteotomy combined with hamate shortening and the Camembert osteotomy combined with ulna shortening; the latter was the only osteotomy that completely unloaded the lunate.

INTERPRETATION

We think the association of the radius Camembert osteotomy and ulna Sennwald's shortening osteotomy is the most effective procedure to propose in Kienböck's disease.

Volar Locking Plate Fixation for Intra-Articular Distal Radius Fractures with Volar Lunate Facet Fragments Distal to the Watershed Line

BACKGROUND

Standard volar plating of distal radius fractures may not adequately fix the volar lunate facet (VLF) fragment, which can result in volar carpal subluxation. We hypothesized that the size of VLF fragments distal to the watershed line might affect reduction loss after distally placed volar locking plate fixation for intra-articular distal radius fracture and examined if the presence of small displaced VLF fragments was a risk factor for reduction loss.

METHODS

Twenty-seven hands of 27 patients with intra-articular distal radius fractures with VLF fragments distal to the watershed line were treated by using Acu-Loc 2 volar distal radius locking plate fixation.

RESULTS

At final follow-up, the mean Mayo Performance Score was 90.9, and the mean Quick Disabilities of Arm, Shoulder, and Hand score was 13.6. On radiography, 5 patients had a reduction loss of >2 mm in ulnar variance from immediately postoperatively to final follow-up (group 1), while 27 had no reduction loss (group 2). The mean longitudinal, transverse, and anteroposterior lengths and joint surface area of the VLF fragment were significantly smaller in group 1 than in group 2. Three-dimensional computed tomography revealed that the fracture patterns of the radiocarpal and distal radioulnar joints in group 1 were mainly volar-displaced VLF fragments.

CONCLUSIONS

By stabilizing fragments, distally placed volar locking plate fixation effectively treated intra-articular distal radius fractures with VLF fragments distal to the watershed line. However, the presence of small displaced VLF fragments may increase the risk of reduction loss in ulnar variance.

Size and stabilization of the dorsoulnar fragment in AO C3-type distal radius fractures

INTRODUCTION

Volar locking plate (VLP) fixation has become the gold-standard treatment for distal radius fractures (DRFs). Especially, internal fixation of the volar lunate facet fragment is essential for the treatment of AO C3-type DRFs. On the other hand, the necessity of the fixation of the dorsal lunate facet fragment (dorsoulnar fragment) remains unclear. The purpose of the present study was to measure three-dimensionally the size of the dorsoulnar fragments in AO C3-type DRFs using computed tomography (CT) images in detail, and to reveal relationships of the size and stabilization of the dorsoulnar fragment with postoperative fracture displacement after VLP fixation.

MATERIALS AND METHODS

We retrospectively reviewed the 101 consecutive Japanese patients who underwent surgical treatment for AO C3-type distal radius fractures. If patient had dorsoulnar fragment, the three-dimensional size of this fragment and the occupying ratio to the radiocarpal joint (RCJ) and the distal radioulnar joint (DRUJ) were anatomically evaluated using the preoperative CT images. In addition, we investigated the relationship of the size and stabilization of the dorsoulnar fragment with fracture displacement after VLP fixation. We statistically compared the size parameters and occupying ratio of the dorsoulnar fragment between the displaced group and the stable groups using a two-tailed t-test. We also statistically compared the numbers of screws inserted into the dorsoulnar fragments between the displaced and stable groups using a chi-square test.

RESULTS

The mean dorsoulnar fragment size was 9.4 mm × 7.9 mm × 11.0 mm and the occupying ratio to the DRUJ and RCJ was 50% and 10%, respectively. The number of patients treated with volar locking plate fixation was 77, of which 12 patients had postoperative displacements. Although the size of the dorsoulnar fragment was not associated with postoperative displacement, stabilization following screw insertion into the dorsoulnar fragment was significantly associated with displacement.

CONCLUSION

Stabilization of the dorsoulnar fragment with at least one screw of the volar locking plate was necessary to prevent postoperative fracture displacement regardless of dorsoulnar fragment size in AO C3-type distal radius fractures.

Radiographic analysis of anatomic risk factors for scaphoid fractures; A case-control study

AIM

The purpose of this study was to investigate the role of anatomic variations in distal radius radiographic indices in patients with or without scaphoid fractures.

MATERIALS AND METHODS

Radial inclination (RI), volar tilt (VT), radial height (RH) and ulnar variance (UV) were measured on wrist radiographs of 320 patients with (Group I, n = 167) or without (Group II, n = 153) scaphoid fracture, fall on outstretched hand (FOOSH). Receiver operating characteristics (ROC) curve analysis was used to assess the diagnostic performance for each variable. Sensitivity (Sn), specificity (Sp), cutoff value, and area under the ROC curve were analyzed. Odds ratio was calculated for defined cutoff values.

RESULTS

The mean age of the groups was similar (29.3 ± 10.2 vs 31.1 ± 9.9 years, p = 0.060). RI (30.0 ± 2.9 vs 26.8 ± 2.3°) VT (11.4 ± 2.4 vs 10.5 ± 2.2°), RH (14.8 ± 2.1 vs 13.2 ± 1.9 mm), UV (-0.46 ± 1.7 vs 0.00 ± 1.5 mm) were higher in scaphoid fracture group (Gr I vs Gr II, p = 0.000, p = 0.000, p = 0.001, p = 0.012 respectively). Ulna minus variant was more prevalent in fracture group (p = 0.001). Optimal cutoff points for RI, VT, RH and UV in differentiating fractured and intact scaphoid were 28.6° (Sn = 81.0%, Sp = 26.3%), 12.2° (Sn = 80.4%, Sp = 67.1%), 14.85 mm (Sn = 80.4%, Sp = 52.1%) and 0 mm (Sn = 88.6%, Sp = 75.8%), respectively. Odds ratios for defined cutoff points for RI, VT, RH and UV were 10.4 (95% CI, 6.2-17.4), 1.8 (95% CI, 1.1-3.0), 3.7 (95% CI, 2.3-6.2) and 2.2 (95% CI, 1.3-3.7) respectively.

CONCLUSION

Increased RI, VT, RH and negative UV were found to be predisposing anatomical risk factors for scaphoid fracture when FOOSH.

Shearing Fracture of Volar Rim of Lunate Fossa of Distal Radius: From Case Report to Literature Review

We report an extremely rare case of isolated fracture of the volar rim of lunate fossa which extends into the sigmoid notch, resulting in distal radioulnar joint (DRUJ) and radiocarpal joint instability. This fracture is easily missed with radiography alone in emergency setting, and should not be treated with casting due to DRUJ instability. (Hong Kong j.emerg.med. 2015;22:118-120)

Development of a biomechanical model of the wrist joint for patient-specific model guided surgical therapy planning: Part 1

An enhanced musculoskeletal biomechanical model of the wrist joint is presented in this article. The developed computational model features the two forearm bones radius and ulna, the eight wrist bones, the five metacarpal bones, and a soft tissue apparatus. Validation of the model was based on information taken from the literature as well as own experimental passive in vitro motion analysis of eight cadaver specimens. The computational model is based on the multi-body simulation software AnyBody. A comprehensive ligamentous apparatus was implemented allowing the investigation of ligament function. The model can easily patient specific personalized on the basis of image information. The model enables simulation of individual wrist motion and predicts trends correctly in the case of changing kinematics. Therefore, patient-specific multi-body simulation models are potentially valuable tools for surgeons in pre- and intraoperative planning of implant placement and orientation.

A Comparison of the Contact Force Distributions on the Acetabular Surface Due to Orthopedic Treatments for Developmental Hip Dysplasia

We used a three-dimensional rigid body spring model (RBSM) to compare the contact force distributions on the acetabular surface of the infant hip joint that are produced by three orthopedic treatments for developmental dysplasia of the hip (DDH). We analyzed treatments using a Pavlik harness, a generic rigid splint, and a spica cast. The joint geometry was modeled from tomography images of a 1-year-old female. The articular cartilage was modeled as linear springs connecting the surfaces of the acetabulum and the femoral head, whereas the femur and the hip bone were considered as rigid bodies. The hip muscles were modeled as tensile-only preloaded springs. The treatments with the Pavlik harness and the generic rigid splint were modeled for an infant in supine position with a hip flexion angle of 90 deg. Also, since rigid splints are often recommended when children are initiating their gait phase, we modeled the treatment with the infant in standing position. For the spica cast, we only considered the infant in standing position with a flexion angle of 0 deg, and the fixation bar at two heights: at the ankle and at the knee. In order to analyze the effect of the hip abduction angle over the contact force distribution, different abduction angles were used for all the treatments modeled. We have found that the treatments with the infant in supine position, with a flexion angle of 90 deg and abduction angles between 60 deg and 80 deg, produce a more homogenous contact force distribution compared to those obtained for the treatments with the infant in standing position.

A biomechanical model of the wrist joint for patient-specific model guided surgical therapy: Part 2

An enhanced musculoskeletal biomechanical model of the wrist joint is presented in this article. The computational model is based on the multi-body simulation software AnyBody. Multi body dynamic musculoskeletal models capable of predicting muscle forces and joint contact pressures simultaneously would be valuable for studying clinical issues related to wrist joint degeneration and restoration. In this study, the simulation model of the wrist joint was used for investigating deeper the biomechanical function of the wrist joint. In representative physiological scenarios, the joint behavior and muscle forces were computed. Furthermore, the load transmission of the proximal wrist joint was investigated. The model was able to calculate the parameters of interest that are not easily obtainable experimentally, such as muscle forces and proximal wrist joint forces. In the case of muscle force investigation, the computational model was able to accurately predict the computational outcome for flexion and extension motion. In the case of force distribution of the proximal wrist joint, the model was able to predict accurately the computational outcome for an axial load of 140 N. The presented model and approach of using a multi-body simulation model are anticipated to have value as a predictive clinical tool including effect of injuries or anatomical variations and initial outcome of surgical procedures for patient-specific planning and custom implant design. Therefore, patient-specific multi-body simulation models are potentially valuable tools for surgeons in pre- and intraoperative planning of implant placement and orientation.

Application of a three-dimensional computational wrist model to proximal row carpectomy

A three-dimensional (3D) computational model of the wrist examined the biomechanical effects of the proximal row carpectomy (PRC), a surgical treatment of certain wrist degenerative conditions but with functional consequences. Model simulations, replicating the 3D bony anatomy, soft tissue restraints, muscle loading, and applied perturbations, demonstrated quantitatively accurate responses for the decreased motions subsequent to the surgical procedure. It also yielded some knowledge of alterations in radiocarpal contact force which likely increase contact pressure as well as additional insight into the importance of the triangular fibrocartilage complex and retinacular/capsular structures for stabilizing the deficient wrist. As better understanding of the wrist joint is achieved, this model could serve as a useful clinical tool.

Post-traumatic carpal instability

The complexity of the carpus explains the difficulty treating carpal injuries. Lesions are dominated by perilunate dislocation, scapholunate dislocation, and scaphoid fractures. The other injuries are trivial. Symptoms include pain and loss of wrist strength, reversible for an acute and well-treated lesion. Too often, these ligament injuries are diagnosed late. For delays longer than 6 weeks, ligament repair is ineffective. These old, complex lesions are potentially highly arthritic in the radiocarpal and mediocarpal joints. Improvements in wrist surgery have mitigated these chronic lesions. Various surgical techniques can preserve a functional wrist; wrist arthrodesis is no longer the only solution for these arthritic wrists. Over the past decade, arthroscopy has contributed to better understanding the injuries of the carpus as well as to better healing them. For acute or chronic ligament injuries without degenerative osteoarthritis, arthroscopy is the treatment of the future. This technique involves a long learning curve and the various arthroscopic techniques must be validated.

Surgical technique of a radial wedge "camembert" osteotomy in Kienböck disease

The main accepted principle to treat Kienböck disease is to decompress the lunate. Radius shortening is the most used technique. Three transverse osteotomies of the radius are described: neutral shortening osteotomy, lateral closing wedge osteotomy, and medial closing wedge osteotomy. Shortening the radius decompress the lunate and more or less the scaphoid. This deviates axial constraints toward ulna and triangular fibrocartilage complex. But the ulnar wrist is not able to support important axial constraints. The authors propose a solution to decompress only the lunate and not the scaphoid. This solution deviates axial constraints toward the scaphoid, which is naturally the most capable bone to support it. The authors describe a new radial nontransverse decompression wedge osteotomy. It allows to shorten the radius in front of the lunate. The fixation is done with a dorsal staple. To complete lunate decompression, authors propose to associate a metaphysal ulnar oblique shortening, essentially if ulnar variance is neutral or positive. The preliminar results on 10 cases are presented.

Validation of radiocarpal joint contact models based on images from a clinical MRI scanner

This study was undertaken to assess magnetic resonance imaging (MRI)-based radiocarpal surface contact models of functional loading in a clinical MRI scanner for future in vivo studies, by comparison with experimental measures from three cadaver forearm specimens. Experimental data were acquired using a Tekscan sensor during simulated light grasp. Magnetic resonance (MR) images were used to obtain model geometry and kinematics (image registration). Peak contact pressures (PPs) and average contact pressures (APs), contact forces and contact areas were determined in the radiolunate and radioscaphoid joints. Contact area was also measured directly from MR images acquired with load and compared with model data. Based on the validation criteria (within 25% of experimental data), out of the six articulations (three specimens with two articulations each), two met the criterion for AP (0%, 14%); one for peak pressure (20%); one for contact force (5%); four for contact area with respect to experiment (8%, 13%, 19% and 23%), and three contact areas met the criterion with respect to direct measurements (14%, 21% and 21%). Absolute differences between model and experimental PPs were reasonably low (within 2.5 MPa). Overall, the results indicate that MRI-based models generated from 3T clinical MR scanner appear sufficient to obtain clinically relevant data.

μCT-generated carpal cartilage surfaces: validation of a technique

Computational models are increasingly being used for the analysis of kinematics and contact stresses in the wrist. To this point, however, the morphology of the carpal cartilage has been modeled simply, either with non-dimensional spring elements (in rigid body spring models) or via simple bone surface extrusions (e.g. for finite element models). In this work we describe an efficient method of generating high-resolution cartilage surfaces via micro-computed tomography (μCT) and registration to CT-generated bone surface models. The error associated with μCT imaging (at 10 μm) was 0.009 mm (95% confidence interval 0.007-0.012 mm ), or ~1.6% of the cartilage thickness. Registration error averaged 0.33±0.16 mm (97.5% confidence limit of ~0.55 mm in any one direction) and 2.42±1.56° (97.5% confidence limit of ~5.5° in any direction). The technique is immediately applicable to subject-specific models driven using kinematic data obtained through in vitro testing. However, the ultimate goal would be to generate a family of cartilage surfaces that could be scaled and/or morphed for application to models from live subjects and in vivo kinematic data.

Finite element modeling of the influence of hand position and bone properties on the Colles' fracture load during a fall

Distal forearm fracture is one of the most frequently observed osteoporotic fractures, which may occur as a result of low energy falls such as falls from a standing height and may be linked to the osteoporotic nature of the bone, especially in the elderly. In order to prevent the occurrence of radius fractures and their adverse outcomes, understanding the effect of both extrinsic and intrinsic contributors to fracture risk is essential. In this study, a nonlinear fracture mechanics-based finite element model is applied to human radius to assess the influence of extrinsic factors (load orientation and load distribution between scaphoid and lunate) and intrinsic bone properties (age-related changes in fracture properties and bone geometry) on the Colles' fracture load. Seven three-dimensional finite element models of radius were created, and the fracture loads were determined by using cohesive finite element modeling, which explicitly represented the crack and the fracture process zone behavior. The simulation results showed that the load direction with respect to the longitudinal and dorsal axes of the radius influenced the fracture load. The fracture load increased with larger angles between the resultant load and the dorsal axis, and with smaller angles between the resultant load and longitudinal axis. The fracture load also varied as a function of the load ratio between the lunate and scaphoid, however, not as drastically as with the load orientation. The fracture load decreased as the load ratio (lunate/scaphoid) increased. Multiple regression analysis showed that the bone geometry and the load orientation are the most important variables that contribute to the prediction of the fracture load. The findings in this study establish a robust computational fracture risk assessment method that combines the effects of intrinsic properties of bone with extrinsic factors associated with a fall, and may be elemental in the identification of high fracture risk individuals as well as in the development of fracture prevention methods including protective falling techniques. The additional information that this study brings to fracture identification and prevention highlights the promise of fracture mechanics-based finite element modeling in fracture risk assessment.

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.

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.

The influence of wire positioning upon the initial stability of scaphoid fractures fixed using Kirschner wires

A finite element model of the carpal scaphoid and its joints was developed to study how wire positioning affects the initial stability of the fixation of scaphoid waist fractures using Kirschner wires. A transverse fracture of the scaphoid waist was simulated along with its fixation using five different two-wire configurations. The resulting models were subjected to a load simulating a 200N force passing through the wrist. Friction between bony fragments was taken into account; as the friction coefficient of cancellous bone is unknown, three different values were analysed. For each of these friction coefficient values, the smallest transverse interfragmentary displacements, and consequently maximum initial stability, were obtained for the model that simulated the maximum gap between wires in the plane of fracture. Results also show that for a similar gap in the plane of fracture, more stable fixation can be achieved when wires cross each other not only in the frontal plane of the hand, but also perpendicularly to it.

A new design concept for wrist arthroplasty

The wrist joint is frequently affected by arthritis, which leads to pain, loss of function and ultimately deformity. Various designs of wrist arthroplasty have been introduced to attempt to relieve pain and provide a functional range of motion. The first generation of wrist implant was a one-piece silicone elastomer. Later generations have designs that have two parts that articulate against each other. However, wrist implants have not achieved the same clinical success to date, compared with hip and knee implants, and there is a high revision rate associated with them. This paper describes a new design concept for wrist arthroplasty, based around the idea of combining the principles of an articulating implant with that of a flexible elastomer implant. The design consists of assembling a radial, carpal/metacarpal, plate and flexible parts together. The radial and carpal/metacarpal parts are to be made from ultra high molecular weight polyethylene. The bearing surfaces of the radial and carpal/metacarpal parts articulate against the flat surfaces of the plate, made from cobalt chrome molybdenum alloy. The radius on the bearing surface of the radial part enables flexion/extension, while the radius on the carpal/metacarpal surface enables radial/ulnar deviation. The articulation of the carpal/metacarpal part against the plate also allows for rotation as well as flexion/extension. The flexible part, made from Elast-Eon, which is a silicone polyurethane elastomer, is inserted through the hole of the plate and into the holes of the radial and carpal/metacarpal parts.

Loss of fixation of the volar lunate facet fragment in fractures of the distal part of the radius

BACKGROUND

The purpose of the present study is to report on a cohort of patients with a volar shearing fracture of the distal end of the radius in whom the unique anatomy of the distal cortical rim of the radius led to failure of support of a volar ulnar lunate facet fracture fragment.

METHODS

Seven patients with a volar shearing fracture of the distal part of the radius who lost fixation of a volar lunate facet fragment with subsequent carpal displacement after open reduction and internal fixation were evaluated at an average of twenty-four months after surgery. One fracture was classified as B3.2 and six were classified as B3.3 according to the AO comprehensive classification system. All seven fractures initially were deemed to have an adequate reduction and internal fixation. Four patients required repeat open reduction and internal fixation, and one underwent a radiocarpal arthrodesis. At the time of the final follow-up, all patients were assessed with regard to their self-reported level of functioning and with use of Sarmiento's modification of the system of Gartland and Werley.

RESULTS

At a mean of two years after the injury, six patients had returned to their previous level of function. The result was considered to be excellent for one patient, good for four, and fair for two. The average wrist extension was 48 degrees, or 75% of that of the uninjured extremity. The average wrist flexion was 37 degrees, or 64% of that of the uninjured extremity. The one patient who underwent radiocarpal arthrodesis had achievement of a solid union. The four patients who underwent repeat internal fixation had maintenance of reduction of the lunate facet fragment. The two patients who declined additional operative intervention had persistent dislocation of the carpus with the volar lunate facet fragment.

CONCLUSIONS

The stability of comminuted fractures of the distal part of the radius with volar fragmentation is determined not only by the reduction of the major fragments but also by the reduction of the small volar lunate fragment. The unique anatomy of this region may prevent standard fixation devices for distal radial fractures from supporting the entire volar surface effectively. It is preferable to recognize the complexity of the injury prior to the initial surgical intervention and to plan accordingly.

Normal ranges of ulnar and radial deviation with reference to ulnar variance

We aimed to determine the normal ranges of radial and ulnar deviation of the wrist in relation to the ulnar variance. A total of 102 healthy subjects (204 wrists) were included in the study. The ranges of radial and ulnar deviation of the wrists were measured using a universal goniometer. Ulnar variance was assessed manually or radiographically, and recorded as ulna minus, ulna plus or ulna minus/plus. When the ranges of radial and ulnar deviation were compared with ulnar variance, ulnar deviation was greater in ulna minus subjects and radial deviation was greater in ulna minus/plus subjects. There was no significant difference in the total range of radio-ulnar deviation. The results of this study indicate that ulnar deviation is greater in ulna minus wrists, and we suggest that ulnar variance should be recorded alongside measurements of radial and ulnar deviation.

Design considerations for a wrist implant

Rheumatoid arthritis and other forms of inflammatory arthritides commonly affect the wrist leading to pain, deformity and a reduced quality of life for the patient. Joint arthroplasty is an attractive solution for improving function while relieving pain, but unfortunately, current designs of wrist arthroplasties have not met with great success. This review paper describes the anatomy and biomechanics of the normal wrist, and reviews the current and past designs of artificial wrist joints. The design considerations for a successful wrist implant are discussed, and it is concluded that future generations of wrist implants should not attempt to recreate the natural wrist, but permit a limited functional range of motion. Different materials and methods of fixation of artificial wrist joints should also be considered to improve implant durability.

Beware of the ulno-palmar distal radial fragment

Five patients sustained isolated fractures of the ulnopalmar rim of the distal radius. In three cases the fragment was small and was classed as an avulsion fracture of the short radiolunate ligament. Palmar wrist subluxation occurred in all cases, four during treatment in a cast or splint. The ulnopalmar rim of the distal radius contributes to the stability of the radiocarpal joint and fractures in this region merit particular attention.