Abstract

Wear is an important factor in the long term success of total knee arthroplasty. Therefore, wear testing methods and machines become a standard in research and implant development. These methods are based on two simulation concepts which are defined in standards ISO 14243-1 and 14243-3. The difference in both concepts is the control mode. One is force controlled while the other has a displacement controlled concept.

The aim of this study was to compare the mechanical stresses within the different ISO concepts. Furthermore the force controlled ISO was updated in the year 2009 and should be compared with the older which was developed in 2001.

A finite element model based on the different ISO standards was developed. A validation calculated with kinematic profile data of the same implant (Aesculap, Columbus CR) in an experimental wear test setup (Endolap GmbH) was done. Based on this model all three different ISO standards were calculated and analysed.

Validation results showed Pearson correlation for anterior posterior movement of 0.3 and for internal external rotation 0.9. Two main pressure maximums were present in ISO 14243-1:2001 (force controlled) with 17.9 MPa and 13.5 MPa for 13 % and 48 % of the gait cycle. In contrast ISO 14243-1:2009 (force controlled) showed three pressure maximums of 18.5 MPa (13 % of gait cycle), 16.4 MPa (48 % of gait cycle) and 13.2 MPa (75 % of gait cycle). The displacement controlled ISO (14243-3:2014) showed two pressure maximums of 16.0 MPa (13 % of gait cycle) and 17.2 MPa (48 % of gait cycle).

The adapted force controlled ISO of the year 2009 showed higher mechanical stress during gait cycle which also might lead to higher wear rates. The displacement controlled ISO leads to higher mechanical stress because of the constraint at the end of the stance phase of the gait cycle. Future studies should analyse different inlay designs within these ISO standards.

Irreparable tendon ruptures constitute a grave clinical problem. Especially for large rotator cuff tears, there often is no primary causal therapy available. As a sad result, the development of a rotator cuff tear arthropathy is more often than not inevitable. Our study investigates the effects of scaffold based tendon regeneration with special focus on mesenchymal stem cells in a rat model.

We used ‘native’ bone marrow stromal cells and cultivated mesenchymal stem cells from male rats that were implanted into female rats. As scaffolds polyglycol acid (PGA) and a collagen I were used. A full-thickness-defect of 2–3 mm in the middle third of the rats achilles tendon was created, which was then filled, with either cell-seeded or not cell-seeded scaffolds and, due to the low primary stability of the scaffolds, fixed with a 4-0 suture. After 12 weeks, a DNA PCR was conducted to verify the existence of male Y-chromosomes in the female regenerated tissue. We determined the maximum tensile load of the regenerated tissue and also did a histological evaluation.

Macroscopically the regenerated tendons were much bigger in diameter, much firmer and also much less elastic than a normal tendon. In the ‘mesenchymal stem cells’ group the implanted cells could be clearly identified after 12 weeks by DNA PCR. The collagen I scaffold yielded better results in the biomechanical study than the PGA scaffold. No evidence of positive influence of the cells on the mechanical stability of the regenerated tissue was found. Collagen I and the use of BMSC histologically lead to increased ossification of the regenerated tissue. In the PGA scaffold group a significant inflammatory reaction was found.

Both scaffold/cell combination seem to be unsuitable for tendon replacement. in-vitro studies on the influence of scaffold material on cell differentiation needs to be done.

One of the recently introduced anchors is the absorbable suture anchor BIOKNOTLESS-RC, a press-fit anchor whose special feature is the knotless reconstruction of the ruptured rotator cuff. We compared the new knotless anchor BIOKNOTLESS-RC with established anchors.

The absorbable pressfit anchor BIOKNOTLESS-RC (DePuyMitek, Raynham, MA, USA), the titanium screw anchor SUPER-REVO 5mm and the tilting anchor ULTRASORB (both ConmedLinvatec, Largo, FL, USA) were tested 12 times in the greater tuberosity of human cadaveric humeri (mean age: 74 years). They were inserted according to the manufacturers instructions with the supplied suture material. An incremental cyclic loading was performed, starting with 75N. Until failure the tensile load was increased by 25N after every 50 cycles. The ultimate failure loads, the anchor displacements and the modes of failure were recorded.

The anchor displacement of the BIOKNOTLESS-RC (15.3mm) after the first cycle with 75N was significantly higher than with the two others (SUPER-REVO 2.1mm, ULTRASORB: 2.7mm). The ultimate failure loads of the tested anchors were comparable: BIOKNOTLESS-RC 150N, SUPER-REVO 150N, ULTRASORB 151N (p> 0,05).

Rupture of the suture material at the eyelet occurred more frequently with the SUPER-REVO. BIOKNOTLESS-RC and ULTRASORB showed a tendency towards anchor pullout.

Our results do not confirm the higher pullout strength of metal anchors, which was found in other studies. Knotless anchors facilitate surgery by eliminating the technically challenging step of arthroscopic knot tying. The disadvantage of the BIOKNOTLESS-RC is its unsatisfactory primary stability. Its initial displacement of a mean of 15.3 mm is clinically significant and jeopardizes the rotator cuff repair.

Because of the high initial displacement and the possible gap formation between tendon and bone, the use of the BIOKNOTLESS-RC in a zone of minor tension, for instance as a second-row anchor in double row technique only is recommend.

Absorbable suture anchors have become more and more important in rotator cuff surgery due to their easy revisability. In osteoporotic bone however they are thought to be of minor primary stability. Purpose of the present study was to compare different absorbable and non-absorbable suture anchors in their pullout strength depending on bone density

The absorbable screw-anchor SPIRALOK5mm (DePuyMitek, Raynham, MA, USA), the titanium screw-anchor SUPER-REVO5mm and the tilting-anchor ULTRASORB (both ConmedLinvatec, Largo, FL, USA) were tested, each anchor representing a different material and design. On the basis of bone density measurement by CT-scans a healthy (mean-age. 42 years) and a osteopenic (mean-age: 74 years) group of cadaveric human humeri were formed. Each anchor was inserted in the greater tuberosity six times. They were inserted according to the manufacturers instructions with the supplied suture material. An incremental cyclic loading was performed, starting with 75N. Until failure the tensile load was increased by 25N after every 50 cycles. The ultimate failure loads, the anchor displacements and the modes of failure were recorded.

In the non-osteopenic bone group, the absorbable SPIRALOK achieved a significantly better pullout strength (mean: 274N) than the titanium screw-anchor SUPER-REVO (mean: 188N) and the tilting-anchor ULTRASORB (mean: 192N). In the osteopenic bone group no significant difference in the pullout strength was found. The failure mechanisms, such as anchor pullout, rupture at eyelet, suture breakage and breakage of eyelet, varied between the anchors. In the osteopenic group the number of anchor pullouts clearly increased.

The present study demonstrates that absorbable suture anchors do not have lower pullout strengths than metal anchors. Depending on their design they can even outmatch metal anchor systems. The results of our study suggest that the anchor design has a crucial influence on primary stability, whereas the anchor material is less important.

In an experimental study in rabbits, bone and cartilage regeneration could be achieved with a new class of resorbable bio-implants. These implants consist of an open porous structure made from polylacitdes and an open porous fleece made from polyglactin/polydioxanon. Both layers were not separated from each other, thus allowing mesenchymal cells to penetrate freely from bone into both the bone substitute and the cartilage substitute layer. It could be shown that ostochondral defects of 4mm diameter and 6mm depth in the condyle of the knee of rabbits healed by the process of mesenchymal cell differentiation into osteocytes and chondrocytes triggered by mechanical load induction only. Evaluation of the newly formed cartilage by light microscopy and immunohistology showed hyaline like features.

However, in many clinical cases chondral defects occur without substantial accompanying bone loss. In these situations, reconstruction of the cartilage defects only seems to be sufficient. However, fixation of such fleeces onto the bone is difficult. On one hand, adherence of the fleece to the underlying bone is crucial, on the other hand an open connection from the bone to the fleece must be accomplished in order to allow mesenchymal cells to penetrate the fleece. Therefor, any kind of glue fixation is not appropriate. To overcome this problem, a new fixation method was developed which allows a safe connection of the fleece onto the bone while providing an open contact of the fleece to the bone marrow for unhampered migration of mesenchymal cells.

The new “Cartilage patches” consist of a fleece (serving as the cartilage substitute layer) made from polyglactin/polydioxanon which had proven its applicability in the above mentioned experiments. Fixation of fleece was achieved by “darts” which were glued onto the fleece. The darts were made from polylacitdes, thus providing sufficient mechanical stability in the bone.

During operation, small holes are cut into the bone by a special instrument. The holes are located in such a way that the darts of the cartilage patch fit into them, such resulting in a stable fixation of the fleece onto the underlying bone. Blood containing mesenchymal cells from the bone marrow is able to flow from the holes into the fleece. In a biomechanical analysis the adherence of the cartilage patches were tested with respect to shear resistance and pull-out stabillity. The results of the tests show that the new cartilage patches withstand the mechanical stress exerted onto articular surfaces and can serve as a new class of cartilage substitute layers. In an animal experiment the applicability of the cartilage patches in reconstruction of cartilage defects in the knee joint of sheep will be proven.