Biomedical Engineering Reference
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strength for osteoporotic cancellous bone. The experimental studies were performed
on five different screws showing that a screw design optimized for pullout strength
is not necessarily optimized also for the insertion torque.
6.2.7
Tapered vs. Cylindrical Screws
Biomechanical studies performed on sheep bones [
44
] showed that the tapered
screws have higher pullout strength and insertion torque in comparison to the paral-
lel screw.
6.3
New Design for a Bioresorbable Interference Screw
6.3.1
Clinical and Retrieval Study
In our clinical practice, the use of bioresorbable screws raised several technical prob-
lems regarding a tendency to pull out of the femoral bone or pull in from the tibial
bone during the early postoperative rehabilitation procedures and a higher risk of
fracture or thread deformation at the screw tip—due to hard bone and an absorption
rate—sometimes incomplete—caused by an improper local blood flow in the bone.
Therefore, the majority of bioabsorbable composite screws, in order to avoid the
screw breakage or damage during insertion, require notching or tapping as a supple-
mentary step for preparing the implantation site. This procedure is inaccurate as it
is performed in a cancellous bone, which causes difficulties in estimating the right
diameter necessary for a stable ligament fixation. Therefore, elimination of the
preparation of the screw insertion site offers a significant advantage by minimizing
the risks of graft damage and decreasing the operation time.
Clinical studies were conducted on a group of 96 patients operated by same team
of surgeons from Colentina Clinical Hospital, Bucharest, with at least 1 year followup,
for Kenneth Jones type of arthroscopic ACL reconstruction: patellar bone— ten-
don—bone auto/allograft; with bioresorbable screws (Storz Megafix and Stryker
Bioabsorbable Interference Screw, PLLA) ( Fig.
6.4
). Patients were followed during a
prospective trial (clinical, X-ray) associated, when possible, with MRI at 6 and
12 months. A standardized Hoffman classi fi cation [
73
] was used to document and
treat possible adverse tissue reaction, with special regard to extra-articular and
intra-articular soft-tissue response (synovial reactions) and to osteolytic lesions.
Retrieval analysis could be performed during two revision cases (rate 2% per year) in
association with core biopsy when accepted by patient at a 1-year follow-up time.
Surface analysis of failed interference screws was made by scanning electron
microscopy (SEM) technique that appears to be a valuable tool for this kind of
analysis, using a Philips XL30 ESEM microscope in the SEM laboratory from
University Politehnica of Bucharest. Stryker instrumentation uses Tibial Coring
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