[Biomechanical investigation on refixation of tuberosities on shoulder prostheses. Does refixation with different suture materials offer enough stability?]. / Biomechanische Untersuchung zur Refixation der Tuberkula an Frakturschulterendoprothesen. Bietet die Rekonstruktion der Tuberkula mittels unterschiedlicher Nahtmaterialien eine ausreichende Stabilität?

BACKGROUND: Elderly patients suffering from complex, non-reconstructable fractures of the proximal humerus are commonly treated by primary implantation of a shoulder endoprosthesis. One of the most critical factors for success or failure of treatment is still the refixation of the tuberosities. METHOD: Using sheep infraspinatus tendons with attached tuberosities three different suture materials were investigated. For 2 of the suture materials 4 tests were accomplished and 5 tests were carried out for the third suture material. A material testing machine was used to perform cyclic loading tests (20 mm/min, Fmin=50 N, Fmax=100 N, respectively after 50 cycles: Fmax+50 N until failure). RESULTS: The results showed large variations in the average maximum forces (152.4 N for suture 1, 219.9 N for suture 2 and 452.3 N for suture 3). All tests showed a high initial lengthening and caused incision-like defects in the bone or tendon and led to failure and high displacement of the tuberosities. CONCLUSION: Due to these results suture materials have a limited usefulness for refixation of tuberosities as an increased risk of obstruction for bony consolidation can result.

Biomechanical strength and failure mechanism of different tubercula refixation methods within the framework of an arthroplasty for shoulder fracture.

BACKGROUND: Tuberosity repair in shoulder fracture prosthesis implantation still remains a challenge often leading to poor functional outcomes, despite a variety of materials and suggested suture patterns. We aimed to evaluate, which forces currently used suture and cerclage materials withstand and to assess whether they are useful with regard to stability of reconstruction of tuberosities and which failure modes they display. MATERIAL AND METHODS: Using sheep infraspinatus tendons with attached tuberosities three different suture materials (suture 1: Ethibond size 2; suture 2: Orthocord size 2; suture 3: Fiberwire size 5) and a 0.8mm titanium cerclage wire were investigated. For each suture material as well as the cerclage wire 6 tests were carried out. A material testing machine was used to perform cyclic loading tests (20mm/min, Fmin=50N, Fmax=100N, respectively after 50 cycles: Fmax+50N until failure). Outcome measures and thus comparison criteria were the maximum holding force, number of cycles reached, total elongation of the system (tendon and suture) and qualitative appraisal and documentation of the mechanism of failure. RESULTS: Overall average maximum forces between the fixation materials differed significantly (P=0.003), especially suture 3 (braided polyethylene coating, non-resorbable polyfile UHMW core) displayed superior results in comparison to the cerclage wire (P=0.016). Although, primary elongation of the cerclage technique was significantly lower compared to the suture materials (P=0.002). All tests showed a high initial lengthening and caused incision-like defects in the bone or tendon and led to failure and huge displacement of the tuberosities. DISCUSSION: Currently used suture and cerclage materials have a limited usefulness for refixation of tuberosities due to an increased risk of obstruction for bony consolidation. LEVEL OF EVIDENCE: Basic science, Biomechanics.

Investigation on stick phenomena in metal-on-metal hip joints after resting periods.

Insufficient understanding of tribological behaviour in total joint arthroplasty is considered as one of the reasons for prosthesis failure. Contrary to the continuous motion input profiles of hip simulators, human locomotion contains motion interruptions. These occurring resting periods can cause stick phenomena in metal-on-metal hip joints. The aim of the present study was to investigate the tribological sensitivity of all-metal bearings to motion interruptions on in vitro test specimens and retrieved implants. Friction and wear with and without resting periods were quantified. Unlike the metal-on-polyethylene joints, the static friction of metal-on-metal joints increased up to micros = 0.3 with rest, while wear appeared to be unaffected. This effect is caused by the interlocking of firmly adhered carbon layers, which were generated from the protein-containing lubricant through tribochemical reactions. Since more than 80 per cent of the retrieved implants exhibited macroscopically visible carbon layers, the increase in friction presumably also occurs under physiological conditions, which is then transferred to the bone-implant interface. These recurrent tangential stress peaks should be considered for the design features of the cup-bone interface, in particular when larger-sized implant heads are used.

[Less is more--a new general practiced oriented form of knee navigation]. / Weniger ist mehr--eine neue praxisorientierte Form der Knie-Navigation.

Modern navigation technology appears to be acquiring an established place in the fields of orthopedics and orthopedic surgery. This technology helps the surgeon to apply his manual skills with greater precision and thus more effectively, and its positive impact on the quality of surgical treatment has already been demonstrated. The SURGETICS navigation system described in this paper shows that the technology can be adapted to the requirements of daily surgical practice, without compromising its utility for the surgeon and, in the last resort, the patient. Provided that due consideration is given to clinical needs, this technology will surely become a standard tool, not only in European hospitals.

[Effect of surface hardness of the femur head prosthesis on abrasive wear forces in pairing with polyethylene for artificial hip joints]. / Einfluss der Oberflächenhärte der Gelenkkugeln auf abrasive Verschleissmechanismen in der Paarung mit Polyethylen für künstliche Hüftgelenke.

The problem of wear has become a major issue in total joint replacement. A correlation between biomaterial hardness and abrasive wear mechanisms may be assumed. To investigate the effect of hardness, cobalt-chromium (CoCr) femoral heads coated with hard amorphous-hydrogenated carbon (a-CH) were tested against uncoated heads under conditions of abrasive wear. The heads were paired with polyethylene (UHMWPE) discs in a ball-on-disc machine. The abrasive wear resistance of the heads increased with surface hardness, and qualitatively differing patterns of wear were observed on the UHMWPE surfaces, depending on the abrasive wear of the matching areas of the heads. Accordingly, when evaluating biomaterials for their suitability for use in total joint replacement, hardness should be considered one of the relevant factors among the material properties with an influence on wear.

[A simplified procedure for mechanical testing of lumbar spinal fixation implants]. / Ein vereinfachtes Verfahren zur mechanischen Prüfung von LWS-Implantaten.

On the basis of the current ASTM and ISO standard proposals, a simplified test procedure for spinal fixation implants has been developed. It comprises static and dynamic tests aimed at evaluating the stiffness and strength of various different internal implants. Different methods of mounting the pedicle screws to the test device are shown to significantly affect the characteristic values and failure mechanisms of the implants. The feasibility of the procedure was investigated by comparing 7 different internal fixation implants. The reproducible results revealed general differences associated with the material, dimensions and design, which latter in particular correlated with the specific failure mechanisms. For longer-term in situ duration, testing of these implants should be expanded to include an analysis of wear and corrosion properties.

The influence of bandage characteristics and inter-individual application variations on underneath bandage pressures.

INTRODUCTION:: Cohesive bandages are applied to the legs of racehorses and horses with limb injuries for protection (prevention of abrasion) and support (reduction of movement at the fetlock joint). The support capacity of all commercially available bandages has been questioned. Consequently, the protection aspect of bandaging and the negative side effects, which can be caused by bandaging (eg pressure induced ischemia with subsequent necrosis), were emphasized. High pressures underneath bandages were shown to cause reduced blood flow. Pressures underneath certain types of bandages were shown to be higher than under others. It is unclear if these differences were due to differences in material characteristics between the bandage types or caused by differences in application by the trainers. The purpose of this study was to determine (a) if differences between different types of bandages are observed if these are applied similarly and (b) if earlier observed differences in pressures underneath bandages are reproducible when wrapped by different trainers. METHODS AND MATERIAL:: (a) A wrapping machine for the application of bandages to an artificial joint (simulating the human knee joint) was designed and built. The machine allows to wrap bandages with adjustable, constant tension under well defined wrapping angles in order to simulate a wrapping technique similar to the wrapping by trainers. The artificial joint is equipped with two pressure sensors (Parotec, Germany). After bandage application, the artificial joint was placed in a specially designed testing machine and cycled through 200 cycles from 0 degrees to 90 degrees of flexion at a frequency of 0.5 Hz. Pressure data were collected at a frequency of 20 Hz. The force required to unwind the bandage from its core was measured with a material testing machine. Five samples each of four different 4" wide cohesive bandages were tested: 'E': high modulus latex, 'V': low mod. latex, 'C': low mod. latex, 'F': medium mod. laminate non-latex. All bandages were applied at manufacturer suggested tensions (50% intercept length). (b) Fifth-three trainers from the Kentucky Horse Center (Lexington, KY) and the Payson Park Training Facility (Indiantown, FL) volunteered for the study. They applied their favoured brand and a type 'F' bandage to an artificial horse limb. Underneath bandage pressures were collected using a pressure mat (Mikro Emed, Novel GmbH, Germany) placed over the fetlock joint. RESULTS AND DISCUSSION:: (a) Maximum pressures during maximum flexion underneath the type 'E' bandage (5.7+/-0.4 N/cm(2)) were significantly higher than for all other bandages (no difference between other bandages, range 3.5+/-0.6 to 4.0+/-0.4 N/cm). The unwinding force for the type 'F' bandage was significantly lower (10.1+/-5.5N) than for all other bandages (range 22.7+/-11.3 to 42.3+/-15.1N). (b) Trainers applied the type 'E' bandage significantly tighter than the other bandages (18.7+/-3.7 N/cm(2) vs a range of 7.5+/-5.4 to 10.4+/-6.1 N/cm(2)); a tendency was observed that the type 'F' bandage was applied looser than the type 'V' bandage. The results of part (b) of this study are consistent with the earlier study. Based on the results of part (a) of this study it can be speculated that (I) The high pressures underneath the type 'E' bandage are clearly due to its material characteristics, and (II) The differences between the other bandages are not caused by material characteristics but probably by the differences in unwinding force. If the unwinding force is higher than the force required to extend the bandage to 50% intercept length, trainers will probably wrap tighter as suggested and desired.

[Comparison of different vertebral body prosthesis with reference to migration and primary stability in dorsoventral spondylodesis after corporectomy with and without laminectomy]. / Vergleich unterschiedlicher Wirbelkörperersatzsysteme hinsichtlich Migration und Primärstabilität bei dorsoventraler Spondylodese nach Korporektomie mit und ohne Laminektomie.

In order to allow speedy mobilization after vertebral body replacement, high primary stability as well as prevention of implant dislocation are required. It is unclear whether differences between common vertebral body replacement systems exist and whether spreadable implants improve primary stability. Migration characteristics of 3 different vertebral body replacement systems were determined using human lumbar spine specimens. Primary stability was assessed by loading specimens successively in 5 conditions: native, after dorsal spondylodesis, after laminectomy and after vertebral body replacement. Migration was shown to depend on cage design and spike shape. Dorsal spondylodesis in combination with vertebral body replacement reduced flexion/extension movement effectively independent of cage design. Stabilization potential in lateral bending was limited. None of the cages limited movement under torsional loading effectively. The possibility to spread the cage had a positive effect on stabilization.

Mismatched wear couple zirconium oxide and aluminum oxide in total hip arthroplasty.

A patient complained about a squeaking noise in his total hip arthroplasty. Clinical evaluation revealed good function, and there were no signs of loosening on the radiograph. Physiotherapy did not alter this phenomenon, and ultimately a revision was performed 42 months after the first surgery. The analysis of the retrievals revealed that a zirconium oxide ceramic head had been paired with a monolithic alumina ceramic cup. The cup showed large deviations from an ideal sphere but minor wear signs. The head exhibited heavy local damage in the articulation zone. This damage might have been caused by the observed unsatisfactory fit between cup and ball, resulting in high stress concentrations and increased wear of the zirconium head. The characteristics of the zirconium and aluminum ceramics pairing might have worsened the process. The combination of implants used in this retrieved wear couple was never approved. To prevent such problems, components of different manufacturers should never be mixed and matched unless explicitly stated.

[Load shift-total hip replacement--numerical and clinical evaluation of a new concept of adaptive stem fixation]. / Die Load Shift-Hüftendoprothese--numerische und klinische Evaluation eines neuen Konzepts der adaptiven Stielverankerung.

AIM: The new load-shift fixation concept pursues the goal of an adaptive change of geometrical characteristics of the in-situ hip replacement. In this study a preclinical and clinical evaluation was performed. METHODS: By three-dimensional, dynamic finite element calculation the biomechanical properties of the prosthesis were analyzed in the early postoperative and long-term configuration. By clinical (20 +/- 10 months) and radiological (18 +/- 10 months) follow-up of a first series of implants (n = 20) the primary functional status of the prosthesis was examined in vivo. RESULTS: The preclinical results revealed that the metaphyseal micromotions are reduced significantly with the diaphyseal fit of the stem postoperatively. By absorption of a biodegradable element (spreader) integrated into the tip of the stem a change of stem geometry associated with loss of the diaphyseal fit occurs. This results in a complete shifting of load transmission into the metaphyseal area. In the clinical situation the postoperative Harris hip score averaged 92.9 +/- 10.5 points. Only in two patients did reversible episodes of thigh pain occur. The radiological analysis of bone remodeling phenomena pointed to a physiological load transmission. There was no evidence of loosening or osteolysis. On radiology, stem tapering could be demonstrated. CONCLUSION: On the basis of these promising preclinical and early clinical results the innovative load-shift prosthesis should now be further evaluated in prospective long-term studies.

[Late complications after alloplastic vascular replacement]. / Spätkomplikationen nach alloplastischem Gefässersatz.

The follow-up of 273 AAA patients operated 1981-1985 showed a mean dilation of polyester grafts of 17.4% in 12 days and 34.8% in 3 years. This early dilation is due to mesh expansion of the warp knitted grafts. It has no clinical relevance. In contrary the late dilation after approximately 10 years is due to degradation. The evaluation of 436 explanted grafts attained from 75 hospitals showed graft rupture to be the cause of the explanation for 42/255 (16%) polyester grafts and 2/42 (5%) PTFE grafts. The most ruptures were observed in the surrounding of the inguinal ligament after 10 to 20 years of duration. In comparison to modern endovascular grafts the conventional polyester and PTFE grafts still are the gold standard of durability.

[Injury of polyester grafts by vascular clamps]. / Schädigung von Polyestergefässprothesen durch Gefässklemmen.

UNLABELLED: Protected vascular clamps are not new. Clamp associated damage of human arteries has already been published over 20 years ago. The necessity of protective clamps seems to have been forgotten. In our explant archive (230 explants) we have observed an accumulation of graft ruptures in the groin (13 of 25 ruptures). We presume a multifactorial process. Clamp damage could be part of it. The aim of this study is to prove the clamp induced damage of polyester vascular grafts and to examine whether protected clamps can reduce this. METHOD: Five unprotected (Aesculap(R) FB512R, FB502, FB517, Ulrich CC1235, CV3535) and 5 protected vascular clamp types (Aesculap(R) FB667, FB668, Edwards(R) - formally Baxter(R) - Fogarty(R) CV5050, CV5201, Edwards(R) Cosgrove(R) CV1033) were tested. A longitudinal burst test was performed after maximal clamp closure on 6 different, multifilament polyester yarns of 2 different vascular grafts manufacturers (B. Braun(R), Edwards(R)). RESULTS: The yarn tests with protected clamps showed no difference to those of the unclamped yarns. After clamping with unprotected vascular clamps the stress-strain-diagrams differed significantly. The mean, maximum burst strength was up to 75 % lower. Video documentation revealed filament ruptures. Damage of the yarn surface was seen on a simple woven graft in scanning electron microscopy (SEM). DISCUSSION: The application of unprotected vascular clamps on polyester vascular grafts is common in Germany (56 %). The observed damage of multifilament polyester yarns makes it necessary to re-consider the use of unprotected vascular clamps. The benefit for biological vessels has already been shown.

Long-term in vivo alterations of polyester vascular grafts in humans.

OBJECTIVES: To examine the influence of in vivo hydrolysis on the physical properties of polyester grafts and their correlation to the period of implantation in the human body. MATERIALS AND METHODS: Sixty-five explanted vascular grafts were obtained after 0-23 years of implantation due to suture aneurysms (18), occlusion (12), graft infection (12), failure of graft material (7) and post-mortem (16). The surface was examined by scanning electron microscopy, the molecular integrity by infra-red spectroscopy and physical strength by probe puncture. RESULTS: Scission of macromolecular chains and loss of strength were shown. It was demonstrated that hydrolytic degradation of polyester takes place with increasing time of implantation in humans. Analysis by linear regression showed that polyester grafts lose 31.4% of their bursting strength in 10 years and 100% in 25-39 years after implantation. CONCLUSIONS: Regular follow-ups of patients with aged vascular grafts and the precise documentation of implanted materials are necessary to estimate graft degradation.