Jochen Erhart

Finite element based estimation of contact areas and pressures of the human scaphoid in various functional positions of the hand.

The scaphoid is the most frequently fractured carpal bone. When investigating fixation stability, which may influence healing, knowledge of forces and moments acting on the scaphoid is essential. The aim of this study was to evaluate cartilage contact forces acting on the intact scaphoid in various functional wrist positions using finite element modeling. A novel methodology was utilized as an attempt to overcome some limitations of earlier studies, namely, relatively coarse imaging resolution to assess geometry, assumption of idealized cartilage thicknesses and neglected cartilage pre-stresses in the unloaded joint. Carpal bone positions and articular cartilage geometry were obtained independently by means of high resolution CT imaging and incorporated into finite element (FE) models of the human wrist in eight functional positions. Displacement driven FE analyses were used to resolve inter-penetration of cartilage layers, and provided contact areas, forces and pressure distribution for the scaphoid bone. The results were in the range reported by previous studies. Novel findings of this study were: (i) cartilage thickness was found to be heterogeneous for each bone and vary considerably between carpal bones; (ii) this heterogeneity largely influenced the FE results and (iii) the forces acting on the scaphoid in the unloaded wrist were found to be significant. As major limitations, accuracy of the method was found to be relatively low, and the results could not be compared to independent experiments. The obtained results will be used in a following study to evaluate existing and recently developed screws used to fix scaphoid fractures.

Anatomy and variants of the triangular fibrocartilage complex and its MR appearance at 3 and 7T.

Due to the small size and complexity of its constituents, the triangular fibrocartilage complex (TFCC) has been a challenging structure for magnetic resonance (MR) imaging. Higher-field MR units, at 3T and 7T, with increased spatial resolution and the development of novel MR sequences, are promising tools for an improved visualization of the ulnocarpal complex. Anatomically, the TFCC consists of the TFC proper, the ulnomeniscal homolog, the ulnar collateral ligament, the ulnotriquetral and ulnolunate ligament, and radioulnar ligaments at the volar (palmar) and the dorsal side, as well as the sheath of the extensor carpi ulnaris tendon and the capsule of the distal radioulnar joint. This article describes the normal anatomy of the TFCC and its appearance on high-field MRI. Anatomical variants, such as the positive ulnar variance, and changes during pronation and supination are addressed.

A finite element analysis of two novel screw designs for scaphoid waist fractures

The scaphoid is the most often fractured carpal bone. Scaphoid fracture repair with a headless compression screw allows for early functional recovery. The rotational stability of a single screw may be limited, having a potential negative impact on the healing process. Two novel screws have been designed to provide improved rotational stability compared to the existing ones. Using a computational finite element model of a scaphoid osteotomy, we compared the efficacy of one simple screw and the two new screws in restricting inter-fragmentary motion (IFM) in three functional positions of the wrist and as a function of inter-fragmentary compression force. The in-plane IFM was primary rotational and was better restricted by the new screws compared to the conventional one when the inter-fragmentary compression force was below 15-20 N, but provided no clear benefit in total flexion independently of the compression force. To better understand the differences in the non-compressed case, we analyzed the acting moments and investigated the effects of the bending and torsional screw stiffness on IFM. By efficiently restricting the inter-fragmentary shear, the new screws may be clinically advantageous when the inter-fragmentary compression force is partially or completely lost and may provide further benefits toward earlier and better healing of transverse waist fractures of the scaphoid.

Cementless two-staged total hip arthroplasty with a short term interval period for chronic deep periprosthetic infection. Technique and long-term results

ZusammenfassungEINLEITUNG: Der zweizeitige Wechsel der Hüftgelenkstotalendoprothese stellt eine anerkannte Methode nach tiefem, chronischem Infekt dar. Allerdings gibt es weder einheitliche Empfehlungen zur Technik der zementfreien Reimplantation noch klinische und radiologische Langzeitergebnisse. MATERIAL UND METHODEN: Unser Protokoll beinhaltet eine Kurzzeitantibiose und eine zementlose Verwendung von Implantaten mit poröser Oberfläche, welche für eine primäre Implantation konzipiert wurden. Zur Anwendung kam diese Methode nach chronischen, tiefen Infekten bei liegender Totalendoprothese des Hüftgelenkes. Klinische und radiologische Langzeitergebnisse von 14 Patienten wurden retrospektiv untersucht. ERGEBNISSE: In einem Nachuntersuchungszeitraum von mindestens fünf Jahren nach Reimplantation der Totalendoprothese kam es bei zwei Patienten zu einer persistierenden Infektion. In drei Fällen kam es zu einer Luxation der Hüfte, welche in zwei Fällen mit einer Reoperation und in einem Fall mit einem Hüft Brace erfolgreich behandelt werden konnten. In keinem der Fälle wurde eine Lockerung der Komponenten beobachtet. Lediglich drei Patienten hatten ein gutes oder sehr gutes, 11 Patienten ein ausreichendes oder schlechtes klinisches Ergebnis. Wir konnten einen signifikanten Zusammenhang zwischen der Länge des Intervals zwischen Explantation und Reimplantation und klinischem Ergebnis feststellen. SCHLUSSFOLGERUNG: Wir gehen von einem deutlich gesteigerten Komfort für den Patienten durch ein kurzes, CRP abhängiges Intervall zwischen Explantation und Reimplantation aus. Das klinisch funktionelle Ergebnis ist signifikant besser je kürzer das Zeitintervall zwischen Explantation und Reimplantation der Prothese ist. Die Reinfektionsrate bzw. Rate an persistierenden Infektionen ist mit derer von zementierten Prothesen nach langfristigem Intervall zwischen Explantation und Reimplantation vergleichbar. Als Vorteil der zementfreien Reimplantation sind ein geringerer Verlust an Knochen und ein besseres knöchernes Einheilverhalten hervorzuheben. Als Grund für die hohe Rate an Luxationen nach Reimplantation sind Veränderungen der Weichteile und der knöchernen Gelenkpfanne zu nennen. Die anhaltend schlechten klinischen Ergebnisse sind durch Glutealinsuffizienz durch die wiederholten operativen Eingriffe und durch Immobilisierung erklärbar, die es möglich kurz zu halten gilt.SummaryBACKGROUND: The two-staged exchange with delayed reimplantation is the most reliable method to treat a deep periprosthetic infection after a total hip arthroplasty (THR). Nether uniform recommendations for the technique of cementless reimplantation, nor long term clinical and radiological results are reported. MATERIALS AND METHODS: Our protocol is performed under short term antibiosis with cementless primary porous hip implants to treat chronic deep periprosthetic infections following THR. A retrospective study was performed to evaluate the clinical and radiological long term outcome and the rate of persistent infection of 14 patients. RESULTS: In a five year minimum follow up persistent infection was observed twice in a two-staged revision THR. A dislocation of the hip components was observed in three cases, and could be surgically treated in twice and by wearing a hip-brace in one case. We could not find a loosening of THR components. Three patients had a good or excellent, 11 patients a fair or bad functional outcome. There was a significant correlation between functional outcome and length of interval of prosthesis exand reimplantation. CONCLUSION: Patients have an increased comfort resulting from a short, CRP depending time interval between ex- and reimplantation. Functional outcome is the better the shorter the interval is. The reinfection rate is comparable to those of cemented revision THRs with a long term interval. We could observe advantages in a reduced amount of bone loss and better osseous integration of the uncemented implants, compared to cemented implants reviewing literature. Alterations of acetabular bone stock and soft tissue are responsible for a high rate of instability of the THR. An inaccurate gait pattern is caused by gluteal weakness in the majority of patients. Repeated surgical intervention and duration of immobilization have to be minimalized.

Rotational Stability of Scaphoid Osteosyntheses: An In Vitro Comparison of Small Fragment Cannulated Screws to Novel Bone Screw Sets

Background The current standard of care for operative repair of scaphoid fractures involves reduction and internal fixation with a single headless compression screw. However, a compression screw in isolation does not necessarily control rotational stability at a fracture or nonunion site. The single screw provides rotational control through friction and bone interdigitation from compression at the fracture site. We hypothesize that osteosyntheses with novel bone screw sets (BSS) equipped with anti-rotational elements provide improved rotational stability. Methods Stability of osteosynthesis under increasing cyclic torsional loading was investigated on osteotomized cadaveric scaphoids. Two novel prototype BSS, oblique type (BSS-obl.) and longitudinal type (BSS-long.) were compared to three conventional screws: Acutrak2®mini, HCS®3.0 and Twinfix®. Biomechanical tests were performed on scaphoids from single donors in paired comparison and analyzed by balanced incomplete random block design. Loading was increased by 50 mNm increments with 1,000 cycles per torque level and repeated until a rotational clearance of 10°. Primary outcome measure was the number of cycles to 10° clearance, secondary outcome measure was the maximum rotational clearance for each torque level. Findings BSS-obl. performed significantly better than Acutrak2®mini and HCS® (p = 0.015, p<0.0001). BSS-long. performed significantly better than HCS® (p = 0.010). No significant difference in performance between BSS-obl. and BSS-long. (p = 0.361), between BSS obl. and Twinfix® (p = 0.50) and BSS long. and Twinfix® (p = 0.667) was detected. Within the torque range up to 200 mNm, four of 21 (19%) BSS-long. and four of 21 (19%) BSS-obl. preparations showed early failure. The same loading led to early failure in four (29%) Twinfix®, seven (50%) Acutrak2®mini and 10 (71%) HCS® of 14 screw samples, respectively. Conclusions For both BSS and to a lesser extent for Twinfix® (as dual-component screw), higher rotational stabilities were identified in comparison to single component headless compression screws.

Unplanned 10-year retention of an external fixator for a proximal tibial fracture.

A 45-year-old homeless male patient did not return for follow-up for routine removal 72 days after an external fixator had been placed for a grade II open fracture of the proximal tibia. Ten years later, he was accidentally referred to our institution for newly incurred fractures of the femoral neck on the same side and also for an opposite-side medial malleolus fracture. The patient was still homeless, with signs of poor personal hygiene, but the original external fixator was in place. Union of his previous proximal tibia fracture had occurred, and there were no signs of any pin-tract infection. Possibilities for this successful outcome include fracture stability, subsequent healing without pin breakage, and 7 weeks of professional wound care before the patients initial discharge from the hospital, resulting in a scarred skin barrier at the pin-skin interface.

Verfahrenswechsel in der operativen Knochenbruchbehandlung der unteren Extremität, geplant und erzwungen

ZusammenfassungDer Verfahrenswechsel kann grundsätzlich auf 2 verschiedenen Überlegungen beruhen. Zum einen kann er geplant sein; dann ist er Teil eines Therapiekonzepts, das darauf ausgerichtet ist, bestimmte Nebenwirkungen und Komplikationen gering zu halten. Zum anderen kann er erzwungen sein, wenn nämlich auftretende Komplikationen den Wechsel eines Implantats erfordern; dann entspricht er einer Reosteosynthese. Die geplanten Verfahrenswechsel an der unteren Extremität haben eine beschränkte Indikation: Sie werden in erster Linie bei instabilen, polytraumatisierten Patienten und bei Frakturen mit einer Tendenz zur Weichteilschwellung und hohen Infektionsraten (Pilon-Tibial-Frakturen) eingesetzt. Weiterhin kommt der Verfahrenswechsel bei Frakturen der langen Röhrenknochen mit schweren Weichteiltraumen zur Anwendung, wobei diese ¶zunehmend primär definitiv mit Hilfe unaufgebohrter, intramedullärer Implantate versorgt werden. Erzwungen wird ein Verfahrenswechsel durch verzögerte oder ausbleibende Knochenbruchheilung, ossäre Defekte, Anwendungsfehler mit dem Implantat und durch Infekte. Der erzwungene Verfahrenswechsel kann einerseits unkompliziert verlaufen, andererseits eine große Bandbreite von Maßnahmen in einem komplizierten, therapeutischen Ablauf erfordern. Dabei richtet sich der Therapiewechsel nach der ¶Ursache für das Misslingen einer Therapie, welche in vollem Umfang verständlich sein muss.AbstractA change of treatment modality in osteosynthesis is based on one of two principles. On the one hand, fixation can be performed as part of a two-step procedure to reduce the side effects of fracture stabilization. On the other hand, a revision osteosynthesis may be required for the treatment of complications. Indications for a two-step procedure are fractures in polytraumatized patients and fractures with a tendency to swelling and ¶infection (pilon-tibial fractures). External fixator devices applied in a two-step procedure were used for stabilization of fractures involving severe soft tissue damage in past decades; unreamed nailing is now commonly preferred. Revision osteosynthesis may be appropriate in the treatment of complications. Such complications can be caused by failure of the osteosynthesis; they can also be due to delayed union or nonunion of fractures, osseous defects or infection. In the case of failed osteosynthesis the reasons for failure must be thoroughly studied and a variety of subsequent procedures considered for successful treatment.

Introduction of a new repair technique in bony avulsion of the FDP tendon: A biomechanical study

The purpose of this study was to determine the biomechanical characteristics of an innovative surgical technique based on a tension banding principle using a suture anchor in the repair of bony avulsions of the flexor digitorum profundus tendon. After injury simulation in 45 fresh frozen distal phalanges from human cadavers, repair was performed with minifragment screws, interosseous sutures and the innovative technique (15 per group). All repairs were loaded for a total of 500 cycles. Subsequently the specimens were loaded to failure. Load at failure, load at first noteworthy displacement (>2 mm), elongation of the system, gap formation at the avulsion site, and the mechanism of failure were assessed. The new techniques’ superior performance in load at failure (mean: 100.5 N), load at first noteworthy displacement (mean 77.4 N), and gap formation (median 0 mm) was statistically significant, which implies a preferable rigidity of the repair. No implant extrusion or suture rupture during cyclic loading were recorded when the new technique was applied. This innovative repair technique is superior biomechanically to other commonly used surgical tendon reattachment methods, particularly with respect to an early passive mobilisation protocol. Further, due to its subcutaneous position, reduction of complications may be achieved.

In vitro experimental investigation of the forces and torque acting on the scaphoid during light grasp

The aim of this study was to measure the magnitude and direction of forces and torque within osteotomized scaphoids within cadaveric wrists during grasping movement of the hand. The mechanical contributions of clinically relevant individual wrist‐crossing tendon groups were investigated. Wrists of eight forearms were immobilized in the sagittal, transverse, and coronal plane on a fixation device with unhindered axial gliding. The scaphoid was osteotomized and the fragments stabilized using an interlocking nail. The nail served as a sensor for measurement of inter‐fragmentary forces orthogonal and torque around the sensor axis. Thus, torque and cantilever forces were measured which originated between the fragments through co‐contraction through the activity of wrist‐crossing tendons. Grasping movement of the hand induced a mean maximum torque of 0.038 ± 0.051 Nm and a force of 4.01 ± 1.71 N on the scaphoid. The isolated activation of thumb tendons resulted in a torque of 9.9 E−3 ± 7.7 E−3 Nm and a force of 1.42 ± 0.49 N. Despite immobilization of the wrist, grasping movement of the hand caused substantial forces and torque within the osteotomized scaphoid bone in varying directions and severity among different specimens. These factors may contribute to the development of nonunions and malunions in unstable scaphoid fractures through interfragmentary micromotion.