Metacarpophalangeal joint mechanics after 3 different silicone arthroplasties

Outcomes of Small Joint Arthroplasty for the Rheumatoid Hand

The number of patients presenting to the hand surgeon with severe rheumatoid disease has decreased with the introduction of more effective medical management. However some rheumatoid patients will undergo disease progression despite this. MCPJ and PIPJ replacement can be effective surgical treatments in these patients but patient selection is crucial. Patients undergo surgery hoping to achieve improvements in pain, hand function and appearance. Multiple types of implant have been introduced over the years; however, silicone remains the most reliable option in terms of survivorship, complications and achieving the aims of rheumatoid patients.

Proximal Interphalangeal Joint Arthroplasty Using a Silicone Implant: A Comparison Between Integra and NeuFlex in 72 Cases

BACKGROUND

Osteoarthritis of the hand can lead to pain, stiffness, and deformation, and thus to functional disability. The purpose of this study was to compare short-term clinical outcomes of 2 silicone proximal interphalangeal (PIP) joint implants, NeuFlex and Integra, in patients with primary osteoarthritis.

METHODS

We included 72 PIP joints, of which 40 were replaced by a NeuFlex implant and 32 by an Integra implant. The average follow-up was 12 months for the Integra group and 16 months for the NeuFlex group.

RESULTS

There was no change in active flexion preoperatively and postoperatively. Extension lag and Disabilities of the Arm, Shoulder, and Hand score decreased substantially in both groups, whereas grip strength and Patient-Specific Functional Scale (PSFS) score increased. All patients were satisfied. Between groups, there was a significant difference in the PSFS score, in favor of the Integra group.

CONCLUSIONS

Both implants have excellent results, but more research is needed with more patients to prevent bias and to determine the long-term outcome of these implants.

The silicone metacarpophalangeal joint arthroplasty: An in-vitro analysis

BACKGROUND

Silicone is still the gold standard implant in metacarpophalangeal arthroplasty. Whereas the clinical results are acceptable, in follow-ups with >10 years, high rates of implant fracture are common, and 5 to 7% of implants required revision. This work's purpose is to analyse the hypothesis that the joint flexion amplitude has a relevant effect on bone strain level, implant stress and bone-implant micromotion, which can reflect an increase in the risk of bone resorption/fatigue failure, implant fracture and osteolysis.

METHODS

To experimentally predict the cortical loading behaviour, composite metacarpals and proximal phalanges were used in intact and implanted states. A finite element model was developed to evaluate the structural behaviour of cancellous bone and implant. This model was validated by comparing cortical strain and load-displacement curve with experimental measurements.

FINDINGS

Bone strain changes between the intact and the implanted states showed a load transfer effect from the cortical to the cancellous bone that increases significantly with the flexion's amplitude rise. The peak implant stress occurred in the flexion amplitudes further away from the implant neutral angle. The highest implant pistoning motion and the highest phalanx cancellous-bone strain occurred simultaneously at the maximum flexion amplitude.

INTERPRETATION

Limiting joint flexion range will be helpful to reduce the strain-shielding effect on cortical bone, minimizing the overload effect on cancellous bone and decreasing the stress levels and the pistoning motion on the implant, ultimately contributing to the longevity of silicone arthroplasty.

Small Joint Arthroplasty of the Hand: An Update on Indications, Outcomes, and Complications

Small joint arthroplasty of the hand is a well-established surgery that can preserve motion and provide reliable pain relief, joint preservation, and improvement in hand function. Soft-tissue integrity is critical in patient and implant selection to avoid postoperative joint instability. Although instability is more common in nonconstrained implants such as pyrocarbon, silicone arthroplasty is associated with high rates of late implant fracture and failure with resultant recurrent deformity and instability. Additional complications such as stiffness, extension lag, and intraoperative fractures may be mitigated by alterations in surgical technique and postoperative rehabilitation protocols. Revision arthroplasty with soft-tissue stabilization procedures have reliable outcomes and can avoid conversion to arthrodesis. This article will review the surgical indications, outcomes of small joint arthroplasty in the hand, and common complications and their management.

In Vivo Metacarpophalangeal Joint Kinematics After Silicone Implant Arthroplasty in Patients With Rheumatoid Arthritis

PURPOSE

The aim of this study was to determine the potential mechanism of implant fracture using 3-dimensional motion analysis of patients with rheumatoid arthritis.

METHODS

Active flexion motion in 9 hands (34 fingers) of 6 female patients with rheumatoid arthritis who previously underwent hinged silicone metacarpophalangeal joint arthroplasty was examined using 4-dimensional computed tomography. Positions of the proximal phalanges relative to the metacarpals were quantified using a surface registration method. The deformation of the silicone implant was classified in the sagittal plane in the maximum flexion frame. The longitudinal bone axis of the proximal phalanx and the helical axis of the proximal phalanx were evaluated in 3-dimensional coordinates based on the hinge of the silicone implant.

RESULTS

Nineteen fingers were classified into group 1, in which the silicone implant moved volarly during flexion without buckling of the distal stem. Twelve fingers were classified into group 2, in which the distal stem of the silicone implant buckled. Three fingers were classified into group 3, in which the base of the distal stem had already fractured. Quantitatively, the longitudinal bone axes of the proximal phalanges were displaced from dorsal to volar in the middle stage of flexion and migrated in the proximal direction in the late phase of flexion. The helical axes of the proximal phalanges were located on the dorsal and proximal sides of the hinge, and these tended to move in the volar and proximal directions as the metacarpophalangeal joint flexed.

CONCLUSIONS

Volar and proximal translation of the proximal phalange was observed on 4-dimensional computed tomography.

CLINICAL RELEVANCE

Proximal displacement of the bone axis late in flexion appears to be a contributing factor inducing implant fractures, because the pistoning motion does not allow the implant to move in the proximal direction.

Treatment of First Carpometacarpal Osteoarthritis with Arthroscopy: A Systematic Review

Background First carpometacarpal (CMC) osteoarthritis or trapeziometacarpal osteoarthritis is a common debilitating hand condition. No one surgical technique has demonstrated superiority in managing this disease. Purpose This study performed a systematic review of arthroscopic techniques for treating first CMC arthritis to assess the effectiveness of different arthroscopic techniques. Methods Grip strength, pinch strength, visual analog scale, the Disability of Arm, Shoulder, and Hand (DASH) score, range of motion (ROM), and complications were recorded. Two subgroup analyses were performed, comparing outcomes of (1) trapeziectomy of any type versus debridement alone and (2) trapeziectomy alone versus interposition versus suspension techniques. Results Preoperative and postoperative scores significantly improved for DASH scores and pain at rest and with activity with variable improvements in ROM. Complications occurred in 13% of cases in publications that reported complications. When comparing studies that utilized techniques with any type of trapeziectomy to debridement alone, only the trapeziectomy subgroup showed significant improvements in pain. When comparing trapeziectomy alone to interposition and suspension techniques, mean DASH scores and pain levels significantly improved in interposition and suspension subgroups. Conclusions The existing literature describes a predominantly female population with Eaton-Littler stage II and III disease. In the subgroup analysis, arthroscopic techniques involving a trapeziectomy seem to be more effective at lowering pain scores compared to techniques involving debridement alone. Likewise, interposition and suspension techniques may show improved outcomes compared to techniques involving trapeziectomy alone. Level of evidence This is a Level III study.

A Systematic Review of Compliant Mechanisms as Orthopedic Implants

Currently available motion-preserving orthopedic implants offer many advantages but have several limitations to their use, including short device lifetime, high part count, loss of natural kinematics, and wear-induced osteolysis and implant loosening. Compliant mechanisms have been used to address some of these problems as they offer several potential advantages - namely, wear reduction, reduced part count, and the ability to achieve complex, patient-specific motion profiles. This article provides a systematic review of compliant mechanisms as orthopedic implants. Based on the PRISMA guidelines for an efficient review, this work identified fourteen implantable orthopedic devices that seek to restore anatomical motion by utilizing mechanical compliance. From reviewing these implants and their results, advantages and consequences for each are summarized. Trends were also identified in how these devices are capable of mitigating common challenges found in orthopedic design. Design considerations for the development of future compliant orthopedic implants are proposed and discussed.

Long-lasting hydrophilic surface generated on poly(dimethyl siloxane) with photoreactive zwitterionic polymers

Poly(dimethylsiloxane) (PDMS) is known as one of the most established polymers for making elastomers. Therefore, it is commonly used for the fabrication of biomedical devices. Many PDMS surface modification processes have been proposed recently to increase PDMS reliability in medical fields. However, the modified surface's long-term stability is still limited. Hydrophobic recovery of PDMS is widely recognized as a factor that reduces the efficacy of PDMS surface modification. The photoreactive zwitterionic polymer effectively suppresses the hydrophobic recovery of PDMS, according to the current analysis. The photoreactive zwitterionic monomer, 2-[2-(Methacryloyloxy)ethyldimethylanmmonium] ethyl benzophenoxy phosphate (MBPP) was polymerized by conventional radical polymerization and coated on O₂-plasma-treated PDMS specimens. The specimens were immersed in an aqueous solution of 2-methacryloyloxyethyl phosphorylcholine (MPC) and exposed under ultraviolet (UV) radiation for 3 h. Instead, of poly(MBPP) (PMBPP), benzophenone (BP) was also used as a conventional photoinitiator. The time-dependent change in the wettability and elemental composition of the specimen surface was monitored for nine weeks after photo-grafting of poly[2-methacryloyloxyethyl phosphorylcholine (MPC)] (PMPC). The advancing and receding contact angles (θ_A/θ_R) of the pristine PDMS specimen were 112°/71° and significantly decreased immediately after the grafting of PMPC regardless of types of photoinitiator. However, the hydrophobicity of the surface gradually recovered, and θ_A was changed from 12° to 81° for nine weeks of storage under air atmosphere when BP was used as a photoinitiator for graft polymerization of MPC. However, surface hydrophilicity (θ_A ≅ 20°) of the surface grafted with PMPC with PMBPP as an initiator was effectively preserved for nine weeks. This surface also showed excellent lubricity and non-fouling properties regardless of the storage periods. Therefore, zwitterionic photoreactive polymer, PMBPP, is then used as a macrophotoinitiator for the surface modification of PDMS.

Quantitative analysis of metacarpophalangeal joints during active flexion using four-dimensional computed tomography

BACKGROUND

The metacarpophalangeal joint has a unique morphology with a high degree of freedom. However, few studies have analyzed the kinematics of fingers owing to the rapid movement of the small bones involved. The in-vivo kinematics of metacarpophalangeal joints were analyzed by four-dimensional computed tomography (4DCT) and associated with its morphology.

METHODS

The flexion motion of the fingers of bilateral hands in 10 volunteers were examined using 4DCT. Iterative surfaces were registered to trace the surface of the proximal phalanges with respect to metacarpals. Rotation angles were calculated using Euler/Cardan angles.

FINDINGS

In the index finger, the proximal phalange supinated to a maximum flexion of 40° and then pronated, and its range of rotation was larger than the previous reports. In the other fingers, the proximal phalanges continued to supinate during flexion. The helical axis of the proximal phalange passed a point extremely close to the center point of bilateral condyles, and it moved toward the proximal and palmar directions until the middle stage of flexion and toward the proximal and dorsal directions during the late stage of flexion. The translation of the rotation axis was larger in the ring and little fingers.

INTERPRETATION

The rotation in the index finger was larger than previously reported. The helical axes moved in the dorsal direction and proximally during the latter phase of the flexion. These results can be employed to better understand the causes of implant failure of the metacarpophalangeal joints.

Factors Associated With Reoperation After Silicone Metacarpophalangeal Joint Arthroplasty in Patients With Inflammatory Arthritis

Background: Silicone metacarpophalangeal (MCP) joint arthroplasty has a high revision rate. It has been suggested that the preoperative degree of ulnar drift and radial wrist deviation influences the durability of MCP silicone arthroplasties. The goal of this study was to evaluate what factors are associated with reoperation after silicone MCP arthroplasty. Materials and Methods: We retrospectively evaluated all adult patients who underwent MCP silicone arthroplasty between 2002 and 2016 at one institutional system for inflammatory arthritis. After manual chart review, we included 73 patients who underwent 252 arthroplasties. Fingers treated included 66 index, 67 long, 60 ring, and 59 small fingers. Results: The overall reoperation rate was 9.1% (N = 23). Indications for reoperation were implant breakage (n = 11), instability (n = 4), soft tissue complications (n = 4), infections (n = 3), and stiffness (n = 1). There was a trend that patients who underwent single-digit arthroplasty had higher rates of revision (19% vs 3.5%, P = .067). Radiographic follow-up demonstrated joint incongruency in 50% of cases, bone erosion in 58% of cases, and implant breakage in 19% of cases. There was a trend toward higher rates of revision in patients without preoperative MCP joint subluxation (19% vs 6.7%, P = .065) The 1-, 5-, and 10-year implant survival rates were 96%, 92%, and 70%, respectively. Revision surgery occurred at <14 months in 15 patients (65%) and after 5 years in 8 (35%) patients. Conclusions: Revision surgery after silicone MCP arthroplasty appears to be bimodal. Patients with greater hand function preoperatively may be at higher risk of revision surgery.

Failure of Osseointegration of a Semiconstrained Finger Prosthesis in a Post-traumatic Metacarpophalangeal Joint Defect: A Case Report

CASE

Post-traumatic defects of the metacarpophalangeal joint (MCPj) remain challenging for hand surgeons. Restoration of stability and motion are difficult to achieve because of the complex anatomy and kinematics. The Robert Mathys (RM) semiconstrained prosthesis offers good intrinsic stability and mobility and therefore seems suitable. We report on a patient where the RM prosthesis was used for a traumatic destroyed ring finger MCPj reconstruction but failed because of aseptic loosening. We discuss possible reasons in light of the current literature on post-traumatic MCPj arthroplasties and the RM prosthesis in particular.

CONCLUSION

The RM proximal interphalangeal prosthesis does not seem suitable for post-traumatic replacement of the MCPj.

[Metacarpophalangeal joint replacement]

BACKGROUND

Arthroplasty of metacarpophalangeal (MCP) joints is crucial for patients with rheumatoid arthritis. Motion preserving therapies are mandatory for this joint, since loss of function of the MCP joint is detrimental. Many protheses or spacers have been introduced over the last 80 years, but most of them have been dismissed due to major complications.

CURRENT PROCEDURES

Since the 1960s the Swanson spacer has been established as the reference standard for motion preserving procedures of the finger MCP joints. High fracture rates of the spacer do not seem to limit function and patient satisfaction after all. Current long-term studies show at least promising results for pyrolytic carbon protheses with respect to range of motion, survival, and revision rates in comparison to the Swanson spacer.

Silicone Metacarpophalangeal Arthroplasty for Osteoarthritis: Long-Term Results

PURPOSE

To demonstrate that silicone metacarpophalangeal (MCP) arthroplasty provides excellent long-term outcomes with a low complication rate in osteoarthritis patients.

METHODS

A consecutive cohort of 35 patients with osteoarthritis of 1 or more MCP joints undergoing anatomically neutral silicone MCP arthroplasty was followed over a 15-year period. Functional outcomes including strength and range of motion (ROM), as well as complications, were recorded. All patients were available for long-term assessment including radiographs and outcomes questionnaire.

RESULTS

Average follow-up for the cohort of 35 patients (40 implants) was 8.3 years (range, 2-17 years). Average age was 58 years (range, 42-80 years) with 22 men and 13 women. In 31 patients, a single MCP joint was involved (middle finger, 20; index finger, 10; little finger, 1). The dominant hand was involved in 23 patients. Seven (of 14) patients had a concomitant RCL reconstruction of the index finger MCP joint; no other digit had a collateral ligament reconstruction. Average final visual analog scale pain score was 0.3 of 10. Average final active ROM arc was from 4° (range, 0°-20°) to 73° (range, 50°-90°) of flexion. One patient had a revision MCP arthroplasty for a 97% clinical survivorship. Radiographs demonstrated fractured implants in 5 of 40 (12.5%) implants, but none exhibited instability, pain, or ROM deterioration. Average Michigan Hand Outcomes Questionnaire score was 82 (of 100) at final follow-up.

CONCLUSIONS

Silicone arthroplasty is effective in the treatment of primary MCP osteoarthritis. Implant survivorship is 97% (clinical) and 88% (radiographic) in long-term follow-up.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic IV.

Effect of accelerated aging on the cross-link density of medical grade silicones

Four specimens of Nagor silicone of different hardness (soft, medium and hard) were swollen, until they reached equilibrium (i.e. constant mass) in five liquids at 25°C, before and after accelerated aging. For the specimens swollen before accelerated aging, the greatest swelling was obtained in methyl cyclohexane, while for the specimens swollen after accelerated aging, the greatest swelling was obtained in cyclohexane. The cross-link density, υ, was also calculated from the swelling measurements for all the specimens, before and after accelerated aging, using the Flory-Rehner equation. The softer silicones, which swelled the most, had lower υ values than harder silicones. The amount of swelling (measured in terms of ϕ) and υ varied significantly (p<0.05) in some cases, between the different silicone hardness and between different liquids. Furthermore, the cross-link density, υ, significantly (p<0.05) increased after accelerated aging in most liquids.Note: ϕ is defined as the volume fraction of polymer in its equilibrium swollen state. A probability value of statistical significance of 0.05 or 5% was selected, hence if a p value of less than 0.05 was obtained, the null hypothesis was rejected (i.e. significant if p<0.05).

Surgical Simulations Based on Limited Quantitative Data: Understanding How Musculoskeletal Models Can Be Used to Predict Moment Arms and Guide Experimental Design

The utility of biomechanical models and simulations to examine clinical problems is currently limited by the need for extensive amounts of experimental data describing how a given procedure or disease affects the musculoskeletal system. Methods capable of predicting how individual biomechanical parameters are altered by surgery are necessary for the efficient development of surgical simulations. In this study, we evaluate to what extent models based on limited amounts of quantitative data can be used to predict how surgery influences muscle moment arms, a critical parameter that defines how muscle force is transformed into joint torque. We specifically examine proximal row carpectomy and scaphoid-excision four-corner fusion, two common surgeries to treat wrist osteoarthritis. Using models of these surgeries, which are based on limited data and many assumptions, we perform simulations to formulate a hypothesis regarding how these wrist surgeries influence muscle moment arms. Importantly, the hypothesis is based on analysis of only the primary wrist muscles. We then test the simulation-based hypothesis using a cadaveric experiment that measures moment arms of both the primary wrist and extrinsic thumb muscles. The measured moment arms of the primary wrist muscles are used to verify the hypothesis, while those of the extrinsic thumb muscles are used as cross-validation to test whether the hypothesis is generalizable. The moment arms estimated by the models and measured in the cadaveric experiment both indicate that a critical difference between the surgeries is how they alter radial-ulnar deviation versus flexion-extension moment arms at the wrist. Thus, our results demonstrate that models based on limited quantitative data can provide novel insights. This work also highlights that synergistically utilizing simulation and experimental methods can aid the design of experiments and make it possible to test the predictive limits of current computer simulation techniques.

Is Swanson prosthesis better than Sutter prosthesis for metacarpophalangeal joint arthroplasty? A meta-analysis

The aim of this meta-analysis is to compare the outcomes of the Swanson and Sutter prostheses (previously the Avanta prosthesis) used for metacarpophalangeal joint arthroplasty, and provide a powerful and rational conclusion regarding the use of prosthesis in MCP joint surgery. The literature search was based on PubMed, Cochrane Library, MEDLINE, EMBASE, and the Chinese National Knowledge Infrastructure. Data were evaluated using a generic evaluation tool designed by the Cochrane Bone, Joint, and Muscle Trauma Group and analysed using RevMan, version 5.0. Six randomised controlled trials were contained in this review, and five of them involving 143 patients were included in the meta-analysis. The results suggested that using the Sutter prosthesis could significantly decrease the rates of recurrence of drift when compared with the Swanson prosthesis for metacarpophalangeal joint arthroplasty (OR = 2.05, 95% Confidence interval (CI) = 1.31-3.20, p = 0.002). No significant difference in the outcomes of prosthesis fracture was found in two groups (OR = 1.07, 95% CI = 0.41-2.79, p = 0.88). Due to the limited data, the outcomes of range of motion, correction of ulnar deviation, pain, grip strength, and radiographic osteolytic changes could not be included in the meta-analyses. Theoretically, recurrence of drift was more common with Swanson prosthesis when compared with the Sutter prosthesis. No significant difference in the outcomes of prosthesis fracture was observed in two groups. More high-quality studies are required in long-term follow-up.

Comparison of Task Performance, Hand Power, and Dexterity with and without a Cock-up Splint

[Purpose] The purpose of this study was to investigate the difference in task performance, grip and pinch strength, and dexterity with and without cock-up splints, which are widely used in occupational therapy practice. [Methods] Twenty-three participants performed Jebsen-Taylor hand function test and grooved pegboard for task performance and dexterity. The power grip and pinch strength was measured using Jamar hydraulic hand dynamometer and pinch gauge. [Results] In the result of the Jebsen-Taylor hand function test, task performance with the cock-up splint was slower compared to without the splint for all items. Men’s grip power with the cock-up splint was found to be significantly decreased compared to without the splint. Women’s grip and palmar pinch strength with the splint decreased significantly compared to without the splint. In the grooved pegboard test, the dexterity of both men and women with the cock-up splint decreased significantly compared to without the splint. [Conclusion] To assist patients to make wise decisions regarding the use of splints, occupational therapists must have empirical knowledge of the topic as well as an understanding of the theoretical, technical, and related research evidence. The results of this study will be useful in the analysis and understanding of changes in hand function in splint applications for people with hand dysfunction.

Proximal interphalangeal joint arthroplasty with silicone implants (NeuFlex) by a lateral approach: a series of 51 cases

The aim of this study is to assess outcomes of a lateral surgical approach for proximal interphalangeal joint arthroplasty with NeuFlex(®) silicone implants for primary degenerative osteoarthritis. We reviewed 51 arthroplasties performed on 43 patients. The mean follow up period was 36 months (range 18-42). The average preoperative range of motion (ROM) was 38°. The average postoperative ROM was 63°. In 21/51 (41%) cases, there was an average axial deviation of 17° (range 10-30°). VAS and Quick DASH scores improved. In 5/51 (10%) cases, further surgery was required. Our lateral approach seems to be effective and minimally invasive, providing adequate exposure. Contralateral incision and contralateral ligament reinforcement should be performed, whenever necessary, to improve lateral stability.

BIOMECHANICS OF THE NORMAL AND DISEASED METACARPOPHALANGEAL JOINT: IMPLICATIONS ON THE DESIGN OF JOINT REPLACEMENT IMPLANTS

The metacarpophalangeal (MCP) joint is crucial for hand function, but is frequently affected by arthritis, leading to pain and disability. This paper reviews the biomechanics of the normal and diseased joint in order to help consider the design of improved MCP joint replacement implants. The normal MCP joint enables a large range of motion in flexion/extension and abduction/adduction as well as a few degrees of rotation. A normal joint typically allows 90° flexion, with a grip strength of up to 672 N. The diseased joint has a reduced range of motion (typically 30° flexion) and reduced hand strength compared to the normal joint. Current MCP joint replacement implants generally try to recreate the range of motion of the normal joint; however, many designs are prone to fracture, as they are unable to withstand the conditions of the diseased joint. It may be beneficial for future implant designs to provide just a functional range of motion. Future designs of MCP joint replacement implants need to be more durable and last longer. Careful consideration of the diseased joint, rather than the normal joint, may help to better define the requirements for such implants.

Improvement of hand function in children with cerebral palsy via an orthosis that provides wrist extension and thumb abduction

BACKGROUND

The present study examines the hand movements of children with cerebral palsy during functional tests and compares the childrens' performance with and without the aid of an orthosis that provides wrist extension and thumb abduction.

METHODS

The range of motion of the trapeziometacarpal joint was assessed for 32 participants via a reflexive markers image system. Observed motions included flexion-extension and abduction-adduction motions performed in the course of four tests for manual ability; the rest position, lateral and tripod pinches and cylindrical grasp. Muscle strength and manual ability were evaluated using dynamometry and the Jebsen-Taylor test.

FINDINGS

The range of motion tests for the rest position, lateral and tripod pinches and cylindrical grasp demonstrated improvements from 17% to 42% (P<0.001) for flexion/extension and from 36% to 54% for abduction/adduction (P<0.001) with the use of the orthosis. Dynamometry measurements showed that the improvement in muscle strength obtained through use of the orthosis was 50% (P<0.001). Improvements in the time required to perform the movements as determined using the Jebsen-Taylor test varied from 13% to 24% (P<0.01) for the four considered tests of manual ability.

INTERPRETATION

The orthosis improved the range of motion of the trapeziometacarpal joint, muscle strength and manual ability. The combination of the three techniques may provide the basis for a quantitative assessment of hand dysfunction/improvement in cerebral palsy that will ultimately guide health professionals in their clinical interventions.

Biomechanical validation of finite element models for two silicone metacarpophalangeal joint implants

Silicone implants are used for prosthetic arthroplasty of metacarpophalangeal (MCP) joints severely damaged by rheumatoid arthritis. Different silicone elastomer MCP implant designs have been developed, including the Swanson and the NeuFlex implants. The goal of this study was to compare the in vitro mechanical behavior of Swanson and NeuFlex MCP joint implants. Three-dimensional (3D) finite element (FE) models of the silicone implants were modeled using the commercial software ANSYS and subjected to angular displacement from 0 deg to 90 deg. FE models were validated using mechanical tests of implants incrementally bent from 0 deg to 90 deg in a joint simulator. Swanson size 2 and 4 implants were compared with NeuFlex size 10 and 30 implants, respectively. Good agreement was observed throughout the range of motion for the flexion bending moment derived from 3D FE models and mechanical tests. From 30 deg to 90 deg, the Swanson 2 demonstrated a greater resistance to deformation than the NeuFlex 10 and required a greater bending moment for joint flexion. For larger implant sizes, the NeuFlex 30 had a steeper moment-displacement curve, but required a lower moment than the Swanson 4, due to implant preflexion. On average, the stress generated at the implant hinge from 30 deg to 90 deg was lower in the NeuFlex than in the Swanson. On average, starting from the neutral position of 30 deg for the preflexed NeuFlex implant, higher moments were required to extend the NeuFlex implants to 0 deg compared with the Swanson implants, which returned spontaneously to resting position. Implant toggling within the medullary canals was less in the NeuFlex than in the Swanson. The differential performance of these implants may be useful in implant selection based on the preoperative condition(s) of the joint and specific patient functional needs.

Acquiring variable moment arms for index finger using a robotic testbed

Human level of dexterity has not been duplicated in a robotic form to date. Dexterity is achieved in part due to the biomechanical structure of the human body and in part due to the neural control of movement. We have developed an anatomically correct testbed (ACT) hand to investigate the importance and behavioral consequences of anatomical features and neural control strategies of the human hand. One of the critical aspects of understanding dexterity is the analysis of the relationships between the hand muscle movements and joint movements, defined by the moment arms of the muscles. It is known that the moment arms for the hand muscles are configuration-dependent and vary substantially with change in posture. This paper presents a methodology for determining continuous variations in the moment arms with respect to multiple joints moving simultaneously. To determine variations in the moment arms of the ACT hand index finger muscles, we employed a nonparametric regression method called Gaussian processes (GPs). GPs give a functional mapping between the joint angles and muscle excursions, and the gradients of these mappings are the muscle moment arms. We compared the moment arm relationships of the ACT hand with those determined from the available cadaver data. We present the implications of the determination of variable moment arms toward understanding of the biomechanical properties of the human hand and for the neuromuscular control for the ACT hand index finger movements.

NeuFlex compared with Sutter prostheses: A blind, prospective, randomised comparison of Silastic metacarpophalangeal joint prostheses

Forty patients (156 metacarpophalangeal joints) with mutilating rheumatoid arthritis were randomly allocated in a blinded manner to have either NeuFlex or Sutter implants. Indications for operation were pain and severe deformity. Thirty-nine patients were followed up postoperatively for one year. An independent physiotherapist and occupational therapist examined each one. Grip strength, range of motion, and pain during activity and at rest were measured. The Canadian Occupational Performance Measure (COPM) assessed the patients' evaluation of their occupational performance. Both groups had overall good results, but it seems that though patients' mobility and grip strength improve considerably, pain seems to do so only relatively. Most patients seem to be satisfied with the operation and their functional gain. Five out of 78 Sutter and two out of 78 NeuFlex implants broke. We found no major differences between the two designs, but the patients in the NeuFlex group seemed to be more satisfied with their occupational performance (COPM performance) (p=0.05).

Osteolysis after Sutter metacarpophalangeal arthroplasty: A prospective study of 282 implants followed up for 5.7 years

Our aim was to evaluate the incidence and degree of osteolysis in a prospective series of patients with rheumatoid arthritis operated on with Sutter implants. Eighty-seven of the 110 operated hands (104 patients) with 282 implants were evaluated after a mean of 5.7 years (2.1-7.4). Osteolytic changes were present in 142 (50%) of the metacarpal and 152 (54%) of the phalangeal bones. Twenty-six of the metacarpal (9%) and 36 of the proximal phalangeal (13%) bones had osteolytic changes that did not affect the cortical bone. Cortical invasion was recorded in 100 (35%) of the metacarpal and 103 (37%) of the proximal phalangeal bones. The cortex was perforated in both bones in 14 (5%). Osteolytic changes were related to fractures of implants and to the dominant hand, but not to pain. Surgeons who operate on patients with rheumatoid diseases should note that silicone rubber implants often cause osteolytic changes.

Frequency dependence of viscoelastic properties of medical grade silicones

Cylinders of medical grade silicone elastomers, (29 mm in diameter and 13 mm thick), immersed in physiological saline solution at 37 degrees C, were investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 +/- 5 N was applied over a frequency range, f, of 0.02-100 Hz. Values of the storage, E', and loss, E'', moduli for the cylinders were found to depend on f; the dependence of E' or E'' on the logarithm (base 10) of f was represented by a third-order polynomial. Above about 0.3 Hz, the cylindrical specimens appeared to be undergoing the onset of a transition from the rubbery to the glassy state. There was no significant difference between results obtained at 37 and 23 degrees C; pretreatment of specimens in physiological saline at 37 degrees C for 24 h and 29 days had no appreciable effect on the results.

Analysis of the mechanism of fracture of silicone metacarpophalangeal prostheses

Eleven fractured Sutter metacarpophalangeal prostheses were obtained from the hands of three patients. All of the implants had fractured at the junction of the distal stem and the hinge. After visual examination, the fracture faces were prepared and viewed using a scanning electron microscope. These images indicated that fracture was initiated by abrasion on the dorsal aspect of the distal stem of the prostheses, close to the hinge. Crack propagation was shown to be due to a fatigue process. Once a crack started, its direction of travel could be followed, using topographical features common to engineering fracture analyses. Propagation was from radial to ulnar and from dorsal to palmar.

Anatomically neutral silicone small joint arthroplasty for osteoarthritis

PURPOSE

Surgical treatment of osteoarthritis at the metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints includes arthrodesis and, more recently, arthroplasty. Silicone joint arthroplasty has been shown to relieve pain and restore function in damaged joints. The purpose of this study was to assess outcomes in patients who had MCP or PIP arthroplasty for osteoarthritis using the NeuFlex implant (DePuy, Warsaw, IN), a silicone implant with an anatomically neutral design.

METHODS

A total of 13 MCP and 16 PIP joint replacements in 19 patients were performed with a mean follow-up period of 4 years (range, 1-8 years). Failed conservative management of osteoarthritis was the indication for surgery in all cases. There were 10 men and 9 women. The average age at the time of surgery was 66 years. Outcomes were assessed by questionnaire evaluation, physical examination, and radiographs.

RESULTS

The mean flexion arc of motion was 65 degrees and 61 degrees , and the mean extension lag was 3 degrees and 0 degrees in the MCP and PIP groups, respectively. Pain relief was rated excellent or good in 84 percent. The overall patient satisfaction was 90 percent. The Michigan Hand Outcomes Questionnaire (MHQ) score at follow-up was 88 in the MCP group and 87 in the PIP group. There was 1 implant fracture in the PIP group.

CONCLUSIONS

At mean 4-year follow-up evaluation, we report high rates of pain relief and patient satisfaction, a low rate of implant fracture, and highly satisfactory range of motion values compared to other implant designs. We show that treatment of MCP and PIP osteoarthritis with an anatomically neutral implant can provide reliable, long-term pain relief and maintenance of function.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic IV.

Survivorship of the Neuflex silicone implant in MCP joint replacement

The Neuflex silicone metacarpophalangeal (MCP) joint replacement has previously been shown to provide improved movement, particularly flexion, when compared with the Swanson implant. In this study, we reviewed the outcome of 237 Neuflex implants in 66 patients with a follow-up of up to 7 years. Kaplan-Meier analysis using revision as the end stage revealed survival at 7 years to be 88%. If however an implant fracture, as seen radiographically, is taken as the end point, the survivorship drops to 68% at 7 years. These figures are comparable with a similar analysis for the Swanson implant. We conclude that the improved range of motion of the Neuflex implant demonstrated previously does not result in either a higher or a lower revision or implant fracture rate when compared with the Swanson implant.

In vivo validation of a realistic kinematic model for the trapezio-metacarpal joint using an optoelectronic system

This article analyzes a realistic kinematic model of the trapezio-metacarpal (TM) joint in the human thumb that involves two non-orthogonal and non-intersecting rotation axes. The estimation of the model parameters, i.e. the position and orientation of the two axes with respect to an anatomical coordinate system, was carried out by processing the motion of nine retroreflective markers, externally attached to the hand surface, surveyed by a video motion capture system. In order to compute the model parameters, prototypical circumduction movements were processed within an evolutionary optimization approach. Quality and reproducibility in assessing the parameters were demonstrated across multiple testing sessions on 10 healthy subjects (both left and right thumbs), involving the complete removal of all markers and then retesting. Maximum errors of less than 5 mm in the axis position and less than 6 degrees in the orientation were found, respectively. The inter-subject mean distance between the two axes was 4.16 and 4.71 mm for right and left TM joints, respectively. The inter-subject mean relative orientation between the two axes was about 106 and 113 degrees for right and left TM joints, respectively. Generalization properties of the model were evaluated quantitatively on opposition movements in terms of distance between measured and predicted marker positions (maximum error less than 5 mm). The performance of the proposed model compared favorably with the one (maximum error in the range of 7-8 mm) obtained by applying a universal joint model (orthogonal and intersecting axes). The ability of in vivo estimating the parameters of the proposed kinematic model represents a significant improvement for the biomechanical analysis of the hand motion.

Finger Kinematic Modeling and Real-Time Hand Motion Estimation

This paper describes methods and experimental studies concerned with quantitative reconstruction of finger movements in real-time, by means of multi-camera system and 24 surface markers. The approach utilizes a kinematic model of the articulated hand which consists in a hierarchical chain of rigid body segments characterized by 22 functional degrees of freedom and the global roto-translation. This work is focused on the experimental evaluation of a kinematical hand model for biomechanical analysis purposes. From a static posture, a completely automatic calibration procedure, based on anthropometric measures and geometric constraints, computes axes, and centers of rotations which are then utilized as the base of an interactive real-time animation of the hand model. The motion tracking, based on automatic marker labeling and predictive filter, is empowered by introducing constraints from functional finger postures. The validation is performed on four normal subjects through different right-handed motor tasks involving voluntary flexion-extension of the thumb, voluntary abduction-adduction of the thumb, grasping, and finger pointing. Performances are tested in terms of repeatability of angular profiles, model-based ability to predict marker trajectories and tracking success during real-time motion estimation. Results show intra-subject repeatability of the model calibration both to different postures and to re-marking in the range of 0.5 and 2 mm, respectively. Kinematic estimation proves satisfactory in terms of prediction capability (index finger: maximum RMSE 2.02 mm; thumb: maximum RMSE 3.25 mm) and motion reproducibility (R (2) coefficients--index finger: 0.96, thumb: 0.94). During fast grasping sequence (60 Hz), the percentage of residual marker occlusions is less than 1% and processing and visualization frequency of 50 Hz confirms the real-time capability of the motion estimation system.

Oxidation of silicone elastomer finger joints

PURPOSE

We analyzed the oxygen content of 19 retrieved implants and 6 packaged implants to further understand the mechanism of degradation of silicone elastomer finger joints while in vivo.

METHODS

Nineteen Swanson (Wright Medical Technology, Arlington, TN) silicone elastomer finger joints were retrieved at revision surgery at an average of 7 years of use. Six packaged and expired implants (5 years after the expiration date) along with the retrieved implants were studied with an elemental analyzer for the total percentage (by weight) of oxygen content.

RESULTS

The retrieved implants showed a mean total oxygen content of 0.41% +/- 0.35%, whereas each of the packaged specimens showed less than 0.1% total oxygen content. Eight of the 19 retrieved implants remained unoxidized. There was no correlation between implant fracture and the oxygen content of the retrieved implants.

CONCLUSIONS

Our results suggest that silicone elastomers are oxidized in vivo. Although such oxygen embrittlement may have implications in compromising silicone elastomer material mechanics during in vivo use, our data indirectly suggests that mechanical factors also have an important role in the final fracture of the silicone elastomer implant.