Biological analysis of flexor tendon repair-failure stump tissue: A potential recycling of tissue for tendon regeneration.

OBJECTIVES: Re-rupture is common after primary flexor tendon repair. Characterization of the biological changes in the ruptured tendon stumps would be helpful, not only to understand the biological responses to the failed tendon repair, but also to investigate if the tendon stumps could be used as a recycling biomaterial for tendon regeneration in the secondary grafting surgery. METHODS: A canine flexor tendon repair and failure model was used. Following six weeks of repair failure, the tendon stumps were analyzed and characterized as isolated tendon-derived stem cells (TDSCs). RESULTS: Failed-repair stump tissue showed cellular accumulation of crumpled and disoriented collagen fibres. Compared with normal tendon, stump tissue had significantly higher gene expression of collagens I and III, matrix metalloproteinases (MMPs), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and insulin-like growth factor (IGF). The stump TDSCs presented both mesenchymal stem and haematopoietic cell markers with significantly increased expression of CD34, CD44, and CD90 markers. Stump TDSCs exhibited similar migration but a lower proliferation rate, as well as similar osteogenic differentiation but a lower chondrogenic/adipogenic differentiation capability, compared with normal TDSCs. Stump TDSCs also showed increasing levels of SRY-box 2 (Sox2), octamer-binding transcription factor 4 (Oct4), tenomodulin (TNMD), and scleraxis (Scx) protein and gene expression. CONCLUSION: We found that a failed repair stump had increased cellularity that preserved both mesenchymal and haematopoietic stem cell characteristics, with higher collagen synthesis, MMP, and growth factor gene expression. This study provides evidence that tendon stump tissue has regenerative potential.Cite this article: C-C. Lu, T. Zhang, R. L. Reisdorf, P. C. Amadio, K-N. An, S. L. Moran, A. Gingery, C. Zhao. Biological analysis of flexor tendon repair-failure stump tissue: A potential recycling of tissue for tendon regeneration. Bone Joint Res 2019;8:232-245. DOI: 10.1302/2046-3758.86.BJR-2018-0239.R1.

The revitalisation of flexor tendon allografts with bone marrow stromal cells and mechanical stimulation: An ex vivo model revitalising flexor tendon allografts.

OBJECTIVES: The present study describes a novel technique for revitalising allogenic intrasynovial tendons by combining cell-based therapy and mechanical stimulation in an ex vivo canine model. METHODS: Specifically, canine flexor digitorum profundus tendons were used for this study and were divided into the following groups: (1) untreated, unprocessed normal tendon; (2) decellularised tendon; (3) bone marrow stromal cell (BMSC)-seeded tendon; and (4) BMSC-seeded and cyclically stretched tendon. Lateral slits were introduced on the tendon to facilitate cell seeding. Tendons from all four study groups were distracted by a servohydraulic testing machine. Tensile force and displacement data were continuously recorded at a sample rate of 20 Hz until 200 Newton of force was reached. Before testing, the cross-sectional dimensions of each tendon were measured with a digital caliper. Young's modulus was calculated from the slope of the linear region of the stress-strain curve. The BMSCs were labeled for histological and cell viability evaluation on the decellularized tendon scaffold under a confocal microscope. Gene expression levels of selected extracellular matrix tendon growth factor genes were measured. Results were reported as mean ± SD and data was analyzed with one-way ANOVAs followed by Tukey's post hoc multiple-comparison test. RESULTS: We observed no significant difference in cross-sectional area or in Young's modulus among the four study groups. In addition, histological sections showed that the BMSCs were aligned well and viable on the tendon slices after two-week culture in groups three and four. Expression levels of several extracellular matrix tendon growth factors, including collagen type I, collagen type III, and matrix metalloproteinase were significantly higher in group four than in group three (p < 0.05). CONCLUSION: Lateral slits introduced into de-cellularised tendon is a promising method of delivery of BMSCs without compromising cell viability and tendon mechanical properties. In addition, mechanical stimulation of a cell-seeded tendon can promote cell proliferation and enhance expression of collagen types I and III in vitro.Cite this article: J. H. Wu, A. R. Thoreson, A. Gingery, K. N. An, S. L. Moran, P. C. Amadio, C. Zhao. The revitalisation of flexor tendon allografts with bone marrow stromal cells and mechanical stimulation: An ex vivo model revitalising flexor tendon allografts. Bone Joint Res 2017;6:179-185. DOI: 10.1302/2046-3758.63.BJR-2016-0207.R1.

Biomechanical analysis of the effect of congruence, depth and radius on the stability ratio of a simplistic 'ball-and-socket' joint model.

OBJECTIVES: The bony shoulder stability ratio (BSSR) allows for quantification of the bony stabilisers in vivo. We aimed to biomechanically validate the BSSR, determine whether joint incongruence affects the stability ratio (SR) of a shoulder model, and determine the correct parameters (glenoid concavity versus humeral head radius) for calculation of the BSSR in vivo. METHODS: Four polyethylene balls (radii: 19.1 mm to 38.1 mm) were used to mould four fitting sockets in four different depths (3.2 mm to 19.1mm). The SR was measured in biomechanical congruent and incongruent experimental series. The experimental SR of a congruent system was compared with the calculated SR based on the BSSR approach. Differences in SR between congruent and incongruent experimental conditions were quantified. Finally, the experimental SR was compared with either calculated SR based on the socket concavity or plastic ball radius. RESULTS: The experimental SR is comparable with the calculated SR (mean difference 10%, sd 8%; relative values). The experimental incongruence study observed almost no differences (2%, sd 2%). The calculated SR on the basis of the socket concavity radius is superior in predicting the experimental SR (mean difference 10%, sd 9%) compared with the calculated SR based on the plastic ball radius (mean difference 42%, sd 55%). CONCLUSION: The present biomechanical investigation confirmed the validity of the BSSR. Incongruence has no significant effect on the SR of a shoulder model. In the event of an incongruent system, the calculation of the BSSR on the basis of the glenoid concavity radius is recommended.Cite this article: L. Ernstbrunner, J-D. Werthel, T. Hatta, A. R. Thoreson, H. Resch, K-N. An, P. Moroder. Biomechanical analysis of the effect of congruence, depth and radius on the stability ratio of a simplistic 'ball-and-socket' joint model. Bone Joint Res 2016;5:453-460. DOI: 10.1302/2046-3758.510.BJR-2016-0078.R1.

Specimen-specific nonlinear finite element modeling to predict vertebrae fracture loads after vertebroplasty.

STUDY DESIGN: Vertebral fracture load and stiffness from a metastatic vertebral defect model were predicted using nonlinear finite element models (FEM) and validated experimentally. OBJECTIVE: The study objective was to develop and validate an FEM-based tool for predicting polymer-augmented lytic vertebral fracture load and stiffness and the influence of metastatic filling materials. SUMMARY OF BACKGROUND DATA: Percutaneous vertebroplasty has the potential to reduce vertebral fracture risk affected with lytic metastases by providing mechanical stabilization. However, it has been shown that the mismatch in mechanical properties between poly(methyl-methacrylate) (PMMA) and bone induces secondary fractures and intervertebral disc degeneration. A biodegradable copolymer, poly(propylene fumarate-co-caprolactone) (P(PF-co-CL)), has been shown to possess the appropriate mechanical properties for bone defect repair. METHODS: Simulated metastatic lytic defects were created in 40 cadaveric vertebral bodies, which were randomized into 4 groups: intact vertebral body (intact), simulated defect without treatment (negative), defect treated with P(PF-co-CL) (copolymer), and defect treated with PMMA (PMMA). Spines were imaged with quantitative computed tomography (QCT), and QCT/FEM-subject-specific, nonlinear models were created. Predicted fracture loads and stiffness were identified and compared with experimentally measured values using Pearson correlation analysis and paired t test. RESULTS: There was no significant difference between the measured and predicted fracture loads and stiffness for each group. Predicted fracture loads were larger for PMMA augmentation (3960 N [1371 N]) than that for the copolymer, negative and intact groups (3484 N [1497 N], 3237 N [1744 N], and 1747 N [702 N]). A similar trend was observed in the predicted stiffness. Moreover, predicted and experimental fracture loads were strongly correlated (R=0.78), whereas stiffness showed moderate correlation (R=0.39). CONCLUSION: QCT/FEM was successful for predicting fracture loads of metastatic, polymer-augmented vertebral bodies. Overall, we have demonstrated that QCT/FEM may be a useful tool for predicting in situ vertebral fracture load resulting from vertebroplasty. LEVEL OF EVIDENCE: N/A.

Intra-articular decorin influences the fibrosis genetic expression profile in a rabbit model of joint contracture.

OBJECTIVES: The goal of this study was to determine whether intra-articular administration of the potentially anti-fibrotic agent decorin influences the expression of genes involved in the fibrotic cascade, and ultimately leads to less contracture, in an animal model. METHODS: A total of 18 rabbits underwent an operation on their right knees to form contractures. Six limbs in group 1 received four intra-articular injections of decorin; six limbs in group 2 received four intra-articular injections of bovine serum albumin (BSA) over eight days; six limbs in group 3 received no injections. The contracted limbs of rabbits in group 1 were biomechanically and genetically compared with the contracted limbs of rabbits in groups 2 and 3, with the use of a calibrated joint measuring device and custom microarray, respectively. RESULTS: There was no statistical difference in the flexion contracture angles between those limbs that received intra-articular decorin versus those that received intra-articular BSA (66° vs 69°; p = 0.41). Likewise, there was no statistical difference between those limbs that received intra-articular decorin versus those who had no injection (66° vs 72°; p = 0.27). When compared with BSA, decorin led to a statistically significant increase in the mRNA expression of 12 genes (p < 0.01). In addition, there was a statistical change in the mRNA expression of three genes, when compared with those without injection. CONCLUSIONS: In this model, when administered intra-articularly at eight weeks, 2 mg of decorin had no significant effect on joint contractures. However, our genetic analysis revealed a significant alteration in several fibrotic genes. Cite this article: Bone Joint Res 2014;3:82-8.

Contrarian behavior in a complex adaptive system.

Contrarian behavior is a kind of self-organization in complex adaptive systems (CASs). Here we report the existence of a transition point in a model resource-allocation CAS with contrarian behavior by using human experiments, computer simulations, and theoretical analysis. The resource ratio and system predictability serve as the tuning parameter and order parameter, respectively. The transition point helps to reveal the positive or negative role of contrarian behavior. This finding is in contrast to the common belief that contrarian behavior always has a positive role in resource allocation, say, stabilizing resource allocation by shrinking the redundancy or the lack of resources. It is further shown that resource allocation can be optimized at the transition point by adding an appropriate size of contrarians. This work is also expected to be of value to some other fields ranging from management and social science to ecology and evolution.

Comparison of step-cut and Pulvertaft attachment for flexor tendon graft: a biomechanics evaluation in an in vitro canine model.

The purpose of this study was to compare two different methods of joining tendons of similar and dissimilar sizes between recipient and donor tendons for flexor tendon grafts. Flexor digitorum profundus (FDP) and peroneus longus (PL) canine tendons were harvested and divided into four groups. The repair technique we compared was a step-cut (SC) suture and a Pulvertaft weave (PW). FDP tendons were significantly larger in diameter than PL tendons (p < 0.05). The volume of the SC repairs using either FDP or PL tendon as a graft was significantly smaller than PW repairs (p < 0.05). The ultimate load to failure and repair stiffness in FDP graft tendons significantly increased compared with the PL graft tendons (p < 0.05). The SC suture can be used as an alternative to the PW, with similar strength and less bulk for repairs using graft tendons of similar diameter. Surgeons should be aware of the effect of graft tendon size and repair method on strength and bulk when performing flexor tendon grafts.

Zona conoidea of the elbow: another articulation between the radial head and lateral trochlear of the elbow.

The zona conoidea comprises the area of the lateral trochlear ridge of the humerus. The purpose of this study is to reintroduce this term 'zona conoidea' to the discussion of the human elbow and to investigate its significance in the development of osteoarthritis of the elbow. The upper extremities of 12 cadavers were prepared. With the forearm in neutral, pronation and supination, the distance between the bevel of the radial head and zona conoidea was inspected. A total of 12 healthy volunteers had a CT scan. The distance between the zona conoidea and the bevelled rim of the radial head was measured in these positions. In the anatomical specimens, early osteo-arthritic changes were identified in the posteromedial bevelled rim of the radial head, and the corresponding zona conoidea in supination. Measurement in the CT study showed that in full supination, the distance between the bevel of the radial head and the zona conoidea was at a minimum. This study suggests that the significant contact between the bevel of the radial head and the zona conoidea in supination is associated with the initiation of osteoarthritis of the elbow in this area.

Retrieval analysis of squeaking ceramic implants: are there related specific features?

INTRODUCTION: Ceramic-on-ceramic total hip arthroplasty is routinely used for young and active patients with end stage of hip osteoarthritis. However, squeaking noise is a recently identified problem with such bearing surface. Many in vivo and in vitro studies have been conducted trying to find the potential causes of this phenomenon. However, we are not aware of any study analyzing retrieved ceramic implants for squeaking. HYPOTHESIS: Our primary hypothesis was that the surface analysis of retrieved ceramic implants with squeaking would present interesting deteriorations that could explain the squeaking noise. MATERIALS AND METHODS: Nine retrieved squeaking implants from ceramic-on-ceramic total hip arthroplasty that were retrieved for various reasons (two exclusively for squeaking, four for recurrent dislocation, one for aseptic loosening and two for instability) were analyzed. Implant positioning was calculated, macroscopic damages were noticed and microscopic roughness was analyzed. The retrieved implants were then tested on a hip simulator reproducing flexion/extension motions in several situations in lubricated and non-lubricated conditions in order to reproduce squeaking. RESULTS: Five cups were considered with borderline insufficient anteversion. Gross impingement damage was visible on seven implants. All the retrieved heads had visible metal transfer on their surface. Eight implants had visible stripe wear. Microscopic analysis showed roughness higher than six microns on the retrieved heads. Squeaking was reproduced in vitro in dry conditions. In lubricated conditions, squeaking did not occur for the retrieved hips. DISCUSSION: This retrieval analysis suggests that problems of cup orientation and design which can lead to impingement can generate lubrication problems because of metal transfer plus/minus stripe wear which is a common theme in ceramic-on-ceramic bearings that squeak.

The contribution of the coronoid and radial head to the stability of the elbow.

We undertook this study to determine the minimum amount of coronoid necessary to stabilise an otherwise intact elbow joint. Regan-Morrey types II and III, plus medial and lateral oblique coronoid fractures, collectively termed type IV fractures, were simulated in nine fresh cadavers. An electromagnetic tracking system defined the three-dimensional stability of the ulna relative to the humerus. The coronoid surface area accounts for 59% of the anterior articulation. Alteration in valgus, internal and external rotation occurred only with a type III coronoid fracture, accounting for 68% of the coronoid and 40% of the entire articular surface. A type II fracture removed 42% of the coronoid articulation and 25% of the entire articular surface but was associated with valgus and external rotational changes only when the radial head was removed, thereby removing 67% of the articular surface. We conclude that all type III fractures, as defined here, are unstable, even with intact ligaments and a radial head. However, a type II deficiency is stable unless the radial head is removed. Our study suggests that isolated medial-oblique or lateral-oblique fractures, and even a type II fracture with intact ligaments and a functional radial head, can be clinically stable, which is consistent with clinical observation.

The effect of core suture flexor tendon repair techniques on gliding resistance during static cycle motion and load to failure: a human cadaver study.

The purpose of this study was to describe a modification of the Massachusetts General Hospital (MMGH) tendon repair and to compare it with three other suture techniques. Twenty human flexor digitorum profundus (FDP) tendons were randomly assigned to the modified Pennington (MP) suture and the MMGH suture. These were compared to the modified Kessler (MK) and Massachusetts General Hospital (MGH) sutures, using data from a previous study. All tendons were repaired with a similar epitendinous stitch and core sutures of 4-0 FiberWire. There was no significant difference in the normalized gliding resistance within the two-strand or four-strand core repair groups. The MP suture had significantly higher 2 mm gap force and ultimate load to failure than the MK suture. The MMGH suture had significantly higher 2 mm gap force and maximum failure ultimate load than the MGH suture. All repairs failed by knot unravelling.

Biomechanical comparison of three fixation techniques used for four-corner arthrodesis.

Clinical results following four-corner arthrodesis vary and suggest that nonunion may be related to certain fixation techniques. The purpose of our study was to examine the displacement between the lunate and capitate following a simulated four-corner arthrodesis with the hypothesis that three types of fixation (Kirschner wires, dorsal circular plate, and a locked dorsal circular plate) would allow different amounts of displacement during simulated wrist flexion and extension. Cadaver wrists with simulated four-corner arthrodeses were loaded cyclically either to implant failure or until the lunocapitate displacement exceeded 1 mm. The locked dorsal circular plate group was significantly more stable than the dorsal circular plate and K-wire groups (p = 0.018 and p = 0.006). While these locked dorsal circular plates appear to be very stable our results are limited only to the biomechanical behavior of these fixation techniques within a cadaver model.

The effects of growth and differentiation factor 5 on bone marrow stromal cell transplants in an in vitro tendon healing model.

The effects of growth differentiation factor-5 (GDF-5) and bone marrow stromal cells (BMSCs) on tendon healing were investigated under in vitro tissue culture conditions. BMSCs and GDF-5 placed in a collagen gel were interpositioned between the cut ends of dog flexor digitorum profundus tendons. The tendons were randomly assigned into four groups: 1) repaired tendon without gel; 2) repaired tendon with BMSC-seeded gel; 3) repaired tendon with GDF-5 gel without cells; and 4) repaired tendon with GDF-5 treated BMSC-seeded gel. At 2 and 4 weeks, the maximal strength of repaired tendons with GDF-5 treated BMSCs-seeded gel was significantly higher than in tendons without gel interposition. However, neither BMSCs nor GDF-5 alone significantly increased the maximal strength of healing tendons at 2 or 4 weeks. These results suggest that the combination of BMSCs and GDF-5 accelerates tendon healing, but either BMSCs or GDF-5 alone are not effective in this model.

Pretreatment of periosteum with TGF-beta1 in situ enhances the quality of osteochondral tissue regenerated from transplanted periosteal grafts in adult rabbits.

OBJECTIVE: To compare the efficacy of in situ transforming growth factor-beta1 (TGF-beta1)-pretreated periosteum to untreated periosteum for regeneration of osteochondral tissue in rabbits. METHODS: In the pretreatment group, 12 month-old New Zealand white rabbits received subperiosteal injections of 200 ng of TGF-beta1 percutaneously in the medial side of the proximal tibia, 7 days prior to surgery. Control rabbits received no treatment prior surgery. Osteochondral transverse defects measuring 5mm proximal to distal and spanning the entire width of the patellar groove were created and repaired with untreated or TGF-beta1-pretreated periosteal grafts. Post-operatively the rabbits resumed normal cage activity for 6 weeks. RESULTS: Complete filling of the defects with regenerated tissue was observed in both the TGF-beta1-pretreated and control groups with reformation of the original contours of the patellar groove. The total histological score (modified O'Driscoll) in the TGF-beta1-pretreated group, 20 (95% Confidence Interval (CI), 19-21), was significantly higher (P=0.0001) than the control group, 18 (16-19). The most notable improvements were in structural integrity and subchondral bone regeneration. No significant differences in glycosaminoglycan or type II collagen content, or equilibrium modulus were found between the surgical groups. The cambium of the periosteum regenerated at the graft harvest site was significantly thicker (P=0.0065) in the TGF-beta1-pretreated rabbits, 121 microm (94-149), compared to controls, 74 microm (52-96), after 6 weeks. CONCLUSIONS: This study demonstrates that in situ pretreatment of periosteum with TGF-beta1 improves osteochondral tissue regeneration at 6-weeks post-op compared to untreated periosteum in 12 month-old rabbits.

A dynamic method for in vitro multisegment spine testing.

Robotics recently spread to spine biomechanical research. The aim of the present work is to describe and validate a new method for in vitro studying of a multisegmental spinal specimen under dynamic conditions. This method relies on the use of a simulator with six degrees of freedom (to impose movements in all directions), an optoelectric apparatus (for collecting kinematics data) and an original system for attaching kinematic markers, allowing their precise removal and replacement under different examination conditions. The accuracy of measurements as well as their reproducibility under static and dynamic conditions is reported here in the study of a human lumbar spinal specimen (L1-sacrum). The method appears to be reliable and reproducible, and should therefore enable future studies of variations in mobility between healthy and pathological spines, to better understand the influence of different implants on spinal kinematics.

The effect of radioscapholunate fusion on wrist movement and the subsequent effects of distal scaphoidectomy and triquetrectomy.

Radioscapholunate arthrodesis is a salvage procedure indicated for osteoarthritis of the radiocarpal joint involving the lunate facet of the radius. This cadaver study examines changes in wrist motion resulting from radioscapholunate arthrodesis, and the effects of surgical techniques to improve the range of motion. Simulated radioscapholunate arthrodesis, distal scaphoidectomy and triquetrectomy were carried out sequentially on six cadaver forearms and measurements (maximum flexion/extension and radial/ulnar deviation) were taken in the intact situation and after each surgical step using a magnetic tracking device. Radioscapholunate arthrodesis diminishes the amplitudes of movements of the wrist in all directions, but range of motion in the radioscapholunate fused wrist improves after scaphoidectomy and improves further after triquetrectomy (88% of original flexion/extension and 98% of original radial/ulnar deviation). Radioscapholunate arthrodesis causes a significant change in kinematics between the hamate and the triquetrum in flexion/extension.

Effects of age and gender on the movement workspace of the trapeziometacarpal joint.

While researchers have suggested that joint mobility would probably be affected by age and gender, research findings often present discrepancies. Little research has been performed on the factors which effect mobility of the trapeziometacarpal (TMC) joint. The purpose of this study was to address the effects of age and gender on the ranges of motion of the normal TMC joint. Eighty normal subjects divided into four age groups participated in this study. The TMC joint motions were recorded using an electromagnetic tracking system. In order to achieve a maximal range of TMC joint motion which was defined as the maximal workspace, each subject was asked to perform actively maximal circumduction, flexion-extension, and abduction-adduction of the TMC joint. Numerical and statistical methods were used to compute the TMC workspace and to detect significant differences. A workspace-to-length ratio was determined as an index to examine the effects of the age and gender on the joint mobility. The results demonstrated that age and gender had significant influences on the TMC workspace among the groups studied. The understanding of TMC joint mobility under different age and gender conditions is achieved through this study. The findings can be used to report clinical measures in the determination of the extent of impairment of osteoarthritis as well as the outcomes between pre- and post-surgical (or non-surgical) interventions.

Prosthesis design and stress profile after hip resurfacing: a finite element analysis.

PURPOSE: To evaluate the effect of prosthesis design on stress profile in the proximal femur after hip resurfacing. METHODS: The von Mises stress profile of the native femur was simulated and compared with that of resurfaced femurs using various prosthetic materials (titanium, cobalt-chrome, ceramic), stem lengths (normal, half, short, and no stem), and femoral head coverage (shell size) [260 degrees, 220 degrees, 180 degrees, and 140 degrees]. RESULTS: Hip resurfacing altered the stress profile of the cancellous (but not cortical) bone of the femoral neck. Maximal cortical stresses were observed at the posterior half of the medial femoral neck. The stress profile of the native femur was most similar to that of the resurfaced femur made of titanium, with a short or no stem and 260 degrees of femoral head coverage (shell size). CONCLUSION: Optimising prosthesis design by minimising biomechanical alterations seems a valid approach to achieving favourable long-term outcomes. Cadaveric and in vivo studies are needed to confirm the clinical relevance and feasibility.

[The Arthur Vick Award: kinematics of the metacarpophalangeal joint after surface replacement arthroplasty]. / Kinematik des Metakarpophalangeal-Gelenks nach Implantation der Oberflächenersatzprothese.

AIM: Prosthetic replacement in the hand must address such unique challenges as preservation of the collateral ligaments, tendon balancing,and Stability. Surface replacement arthroplasty can be an alternative to other current implants. The purpose of this study was to evaluate the metacarpophalangeal joint kinematics after surface replacement arthroplasty. METHOD: The kinematics of pyrolytic carbon as a surface replacement implant for the metacarpophalangeal joint (MCP) was compared with the intact MCP joint in eight fresh cadaver long fingers by means of an electromagnetic tracking system (Polhemus, Colchester, VT). The eight human cadaver MCP joints were tested before implantation, after implantation, after collateral ligaments resection, and after collateral ligaments reconstruction. RESULTS: The kinematics of the MCP joint is reproduced by the joint surface replacement arthroplasty when normal ligament tension was present. The maximum angular displacement of the pyrocarbon implant was 378 for lateral deviation and 338 for rotation during the passive flexion and extension motion. The instantaneus center of rotation (ICR) after implant insertion was nearly identical to the center of rotation of the normal joint. The results also indicated that the collateral ligaments provide the primary stability of the MCP joint. No significant differences in lateral and rotational stability after surface replacement arthroplasty were noted. While collateral ligaments resection significantly affected the stability of the MCP joint. CONCLUSION: The ICR of the pyrocarbon implant most closely matched that of the intact MCP joint. The pyrocarbon implant provides suitable stability to radio-ulnar deviation and rotational stresses as a resurfacing implant and it simulates the kinematics of the intact MCP joint. By using new materials and taking the anatomical and biomechanical requirements into consideration, the endoprosthesis of the finger joints has created an option to achieve good long-term results. The inadequate results of earlier and current prostheses are a consequence of their mechanical construction and their materials. The success of the new implants could be proven by preferably long-term, controlled studies.

Scleral penetration force requirements for commonly used intravitreal needles.

INTRODUCTION: Repeated intraocular injections have become routine practice. It is important to determine how to make them as painless as possible. We wished to examine the force required to penetrate the sclera with different gauge needles. METHODS: Recently enucleated eyes had 4 mm scleral punch biopsies performed. The scleral disks were mounted on a fixed load cell and either a new needle or a needle that had previously penetrated a rubber vial was attached to a linear stage and the maximum penetration force measured. RESULTS: The 27-gauge needles required almost twice as much force to penetrate the sclera than either the 30- or the 31-gauge. In addition, the 30- and 31-gauge appeared to require similar force. In all cases, following vial perforation, there was an increase in the amount of force required to penetrate the sclera. The amount of force was variable. DISCUSSION: Smaller gauge needles require less force to penetrate the sclera and needles that have been used to penetrate rubber vials require more force.