Hakim Louati

In vitro assessment of strength, fatigue durability, and disassembly of Ti6Al4V and CoCrMo necks in modular total hip replacements.

Modularity in total hip replacement offers advantages with regard to biomechanical adjustments and leg lengths. Recently, modular femoral necks were introduced as an added advantage to head modularity permitting further adjustments in femoral version as well as offset and ease of revision. Currently, most necks are made of Ti6Al4V for which cases of in vivo fractures and inseparable neck-stem junctions have been reported. Therefore, we investigated CoCrMo head-Ti6Al4V stem hip replacements with necks made of CoCrMo as an alternative to Ti6Al4V. We compared the two materials with respect to (1) compressive load bearing capacity; (2) fatigue durability; and (3) component distraction. We performed in vitro fatigue-pull-off, microscopy, fatigue durability and compression investigations. The CoCrMo neck showed a load bearing capacity of 18 kN, 38% higher than 13 kN for the Ti6Al4V neck. A fatigue load of 11.2 kN for 1 million cycle failure was achieved with CoCrMo translating into nearly 1000 times longer fatigue life compared to Ti6Al4V necks. The neck-stem distraction force showed large statistical variation and was similar for both neck materials. Overall, the results suggest a superiority of CoCrMo over Ti6Al4V as neck material with regard to mechanical behavior. However, the corrosion behavior was not appropriately assessed and necessitates additional investigations.

Proper placement of the distal biceps tendon during repair improves supination strength—a biomechanical analysis

BACKGROUND Anatomic repair of the distal biceps tendon can be difficult to achieve. This study was designed to compare the effect of anatomic and nonanatomic repairs on forearm supination torque. A nonanatomic repair re-establishes the footprint radial and more anterior to the tuberosity apex, whereas an anatomic repair re-establishes the footprint ulnar and more posterior to the tuberosity apex. METHODS Eight fresh frozen cadaver arms were surgically prepared and mounted on an elbow simulator. Controlled loads were applied to the long head and short head in positions of pronation, neutral, and supination. This was done with intact tendons and then repeated with repaired tendons that were repaired either anatomically (ulnar position) or nonanatomically (radial position). RESULTS All anatomic repairs showed no difference compared with intact tendon measurements. In comparing anatomic and nonanatomic repairs, we found no differences in the supination torque when the forearm was in 45° of pronation. However, when the arm was in neutral rotation, we found that 15% less supination torque was generated by the nonanatomic repair. When the arm was tested in 45° of supination, we found that 40% less supination torque was generated in the nonanatomic repair (P = .01). CONCLUSION This study supports the idea that an anatomic repair of the biceps tendon onto the ulnar side of the radial tuberosity is important. If the tendon is repaired too radially, the biceps will lose the cam effect and may not be able to generate full supination torque when the forearm is in neutral rotation or in supination.

Retrieval analysis and in vitro assessment of strength, durability, and distraction of a modular total hip replacement†

We investigated a commercial Co-Cr-alloy head--Ti6Al4V alloy neck and Ti6Al4V stem modular total hip replacement. We assessed the distraction forces after in vitro cycling in bovine serum, fatigue durability, fretting corrosion damage, and load bearing capacity of new implants using fatigue-corrosion, pull-off, scanning electron microscopy, fatigue and compression investigations. In addition, we studied corrosion, fretting damage, and distraction forces on retrievals. For both retrievals and in vitro test samples, the neck-stem interface required the higher distraction force as compared with the head-neck interface. One of 12 retrievals showed strong fretting corrosion at the neck-stem interface which resulted in a high disassembly force of about 16 kN. For in vitro test samples, the neck-stem pull-off force initially increased during cycling and showed a maximum value of 5.704 kN at ∼100,000 cycles, which is equivalent to gait cycles performed in approximately 36 days. Overall, assembly force, initial component settling, and interface corrosion primarily determine the force required to distract the modular components. One million cycles fatigue failure of the neck can be expected at a maximum compression load of -6.5 kN. No component failure was observed during quasistatic compression; rather the neck deformed plastically and the ultimate compression load-bearing capacity was -13 kN.

Effects of hip implant modular neck material and assembly method on fatigue life and distraction force

Hip implant neck fractures and adverse tissue reactions associated with fretting‐corrosion damage at modular interfaces are a major source of concern. Therefore, there is an urgent clinical need to develop accurate in vitro test procedures to better understand, predict and prevent in vivo implant failures. This study aimed to simulate in vivo fatigue fracture and distraction of modular necks in an in vitro setting, and to assess the effects of neck material (Ti6Al4V vs. CoCrMo) and assembly method (hand vs. impact) on the fatigue life and distraction of the necks. Fatigue tests were performed on the cementless PROFEMUR® Total Hip Modular Neck System under two different loads and number of cycles: 2.3 kN for 5 million cycles, and 7.0 kN for 1.3 million cycles. The developed in vitro simulation setup successfully reproduced in vivo modular neck fracture mode and location. Neck failure occurred at the neck–stem taper and the fracture ran from the distal lateral neck surface to the proximal medial entry point of the neck into the stem. None of the necks failed under the 2.3 kN load. However, all hand‐assembled Ti6Al4V necks failed under the 7.0 kN load. In contrast, none of the hand‐assembled CoCrMo necks and impact‐assembled necks (Ti6Al4V or CoCrMo) failed under this higher load. In conclusion, Ti6Al4V necks were more susceptible to fatigue failure than CoCrMo necks. In addition, impact assembly substantially improved the fatigue life of Ti6Al4V necks and also led to overall higher distraction forces for both neck materials. Overall, this study shows that the material and assembly method can affect the fatigue strength of modular necks. Finally, improper implant assembly during surgery may result in diminished modular neck survivability and increased failure rates.

The Effect of Varying Tension of a Suture Button Construct in Fixation of the Tibiofibular Syndesmosis—evaluation Using Stress Computed Tomography

BACKGROUND/PURPOSE There have been no studies assessing the optimal biomechanical tension of suture button constructs. The purpose of this study was to assess optimal tensioning of suture button fixation and its ability to maintain reduction under loaded conditions using a stress computed tomography (CT) model. METHODS Ten cadaveric lower limbs disarticulated at the knee were used. The limbs were placed in a modified ankle load frame that allowed for the application of sustained torsional axial or combined torsional/axial loads. The syndesmosis and the deep deltoid ligaments complex were sectioned and the limbs were randomized to receive a suture button construct tightened at 4, 8, or 12 kg. The specimens were loaded under the 3 loading scenarios with CT scans performed after each and at the conclusion of testing. Multiple measurements of translation and rotation were compared with baseline CT scan taken before sectioning. RESULTS Significant lateral (maximum 5.26 mm) and posterior translation (maximum 6.42 mm) and external rotation (maximum 11.71 degrees) was noted with the 4 kg repair. Significant translation was also seen with both the 8 and the 12 kg repairs; however, the incidence was less than with the 4 kg repair. Significant overcompression (ML = 1.69 mm, B = 2.69 mm) was noted with the 12 kg repair and also with the 8 kg repair (B = 2.01 mm). CONCLUSION Suture button constructs must be appropriately tensioned to maintain reduction and re-approximate the degree of physiological motion at the distal tibiofibular joint. These constructs also demonstrate overcompression of the syndesmosis; however, the clinical effect of this remains to be determined.BACKGROUND/PURPOSE There have been no studies assessing the optimal biomechanical tension of suture button constructs. The purpose of this study was to assess optimal tensioning of suture button fixation and its ability to maintain reduction under loaded conditions using a stress computed tomography (CT) model. METHODS Ten cadaveric lower limbs disarticulated at the knee were used. The limbs were placed in a modified ankle load frame that allowed for the application of sustained torsional axial, or combined torsional/axial loads. The syndesmosis and the deltoid ligament complex were sectioned and the limbs were randomized to receive a suture button construct tightened at 4kg, 8kg, or 12kg. The specimens were loaded under the 3 loading scenarios with CT scans performed after each as well as at the conclusion of testing. Multiple measurements of translation and rotation were compared to baseline CT scan taken prior to sectioning. RESULTS Significant lateral (maximum 5.26mm) and posterior translation (maximum 6.42mm) as well as external rotation (maximum 11.71°) was noted with the 4kg repair. Significant translation was also seen with the both the 8kg and 12 kg repairs however the incidence was less than with the 4kg repair.Significant overcompression (ML=1.69mm, B=2.69mm) was noted with the 12kg repair and also with the 8kg repair (B=2.01mm). CONCLUSION Suture button constructs must be appropriately tensioned to maintain reduction and re-approximate the degree of physiological motion at the distal tibiofibular joint. These constructs also demonstrate overcompression of the syndesmosis however the clinical effect of this remains to be determined.

Lateral Elbow Exposures: The Extensor Digitorum Communis Split Compared with the Kocher Approach

Introduction In comparison with the frequently used modified Kocher approach, the extensor digitorum communis (EDC) splitting approach allows improved access to the anterior half of the radial head, which is most commonly fractured, while reducing the risk of iatrogenic injury to the lateral collateral ligament. Step 1 Make the Incision Modified Kocher Approach Make an oblique 7-cm lateral incision beginning at the proximal edge of the lateral epicondyle and extending distally over the center of the radial head toward the posterior ulnar border of the extensor carpi ulnaris muscle belly. Step 2 Develop the Interval Between the Anconeus and the Extensor Carpi Ulnaris Identify and develop the intermuscular interval between the anconeus and the extensor carpi ulnaris. Step 3 Perform the Lateral Elbow Capsulotomy Longitudinally incise the lateral elbow capsule and annular ligament anterior to the lateral ulnar collateral ligament. Step 4 The Extended Modified Kocher Approach Extend the exposure by elevating the common extensor origin (extensor carpi radialis brevis, EDC, and extensor carpi ulnaris) proximally off the lateral epicondyle and reflect it anteriorly. Step 5 Make the Incision EDC Splitting Approach Make a longitudinal oblique 5 to 6-cm lateral incision beginning at the proximal edge of the lateral epicondyle and extending distally over the radial head toward the Lister tubercle. Step 6 Identify and Split the EDC The EDC tendon is identified and bisected longitudinally starting proximally at its origin on the lateral epicondyle and extending 20 mm distally from the radiocapitellar joint. Step 7 Perform the Lateral Elbow Capsulotomy The annular ligament and joint capsule are then incised collinear with the EDC split anterior to the equator of the capitellum. Step 8 Extended EDC Splitting Approach Extend the exposure by detaching the anterior half of the EDC tendon and the extensor carpi radialis brevis tendon from the lateral epicondyle. Step 9 Layered Closure Perform an interrupted layered closure. Results In our recent cadaveric study, we quantitatively compared the modified Kocher and EDC splitting approaches in order to determine which provided the greatest exposure of the anterior aspect of the radial head, which is most commonly fractured.IndicationsContraindicationsPitfalls & Challenges.