A Novel Free-Floating Polycarbonate-Urethane Meniscal Implant: From Bench to Clinical Practice
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Elsner JJ, 1Zur G, 2Condello V, 2 Zorzi C, 3Hershman E, 4Guilak F, 1Shterling A, 1 Linder-Ganz E
+1Active Implants Corporation, R&D Center, Netanya, Israel; 2Sacro Cuore Don Calabria Hospital, Verona, Italy; 3Lenox Hill Hospital, New York, NY;
4
Duke University Medical Center, Durham, NC
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Corresponding author [email protected]
INTRODUCTION:
The menisci are semi-lunar wedge-shaped structures that play critical roles
in load distribution, shock absorption, and joint congruity in the knee.
Meniscal tears are common knee injuries that subsequently lead to
degenerative arthritis, attributed primarily to the changes in the magnitude
and pattern of stress distribution in the knee. In such cases there is clearly a
need to protect the articular cartilage by either repairing or replacing the
menisci. Traditionally, meniscal replacement involves implantation of an
allograft. However, besides problems related to availability, size matching,
cost and risk of disease transmission, allograft menisci undergo remodeling
after implantation, causing shrinkage and reduced mechanical strength.
Artificial prostheses offered thus far are based principally on tissue
engineering concepts. The variability in the body response to biodegradable
implants and the quality of the tissue formed still pose a problem in this
respect, thus making it difficult to attain satisfying results from scaffold-type
meniscal implants under intense knee loading conditions, and especially in
patients >40 years old. Therefore, the goal of this study was, to develop a biostable synthetic meniscal implant which combines durability with a
dependable biomechanical performance resembling that of the natural
meniscus. This implant could accommodate patients > 40 years old for whom
biological solutions are not satisfactory.
Figure #1: the composite
polycarbonate-urethane and
UHMWPE meniscal implant
developed in the study
METHODS:
A composite, self-centering, non-fixed discoid-shaped meniscus implant
composed of polycarbonate-urethane (PCU, "Bionate", PTG-DSM), a
compliant yet durable polymer, reinforced circumferentially with UHMWPE
fibers (Dyneema Purity®, DSM) is proposed (Fig. #1). The implant shape is
based on an extensive MRI study that includes more than 100 knee scans [1].
The proposed structure aims to mimic the function of collagen fibers, which
are arranged predominantly in the circumferential direction, within a hydrated
matrix. This configuration has been shown to support hoop stresses,
optimizing distribution of contact stresses within the knee joint and prevent
meniscal extrusion [2]. Biomechanical evaluation of the implant was focused
on in-vitro measurements of contact pressure distributions under the implant
in cadaver knees [3] and computational finite element (FE) analyses [4].
Pressure distribution on the tibial plateau (under the meniscus implant) was
measured by pressure sensitive films (Tekscan, MA) and quantified with
respect to the natural meniscus. The effects of geometrical and material
properties of the composite structure were investigated as factors influencing
its pressure distribution ability, specifically focusing on variations in the type
of reinforcing material and its distribution. FE analyses were used to evaluate
internal stress and strains, and to support the selection of optimal implant
configuration. Additionally, dynamic fatigue tests (15-million cycles) were
conducted to evaluate long-term stability of the implant structure under cyclic
loading. The last pre-clinical step was a large-animal study in which the
condroprotective effect of the implant design was investigated in a sheep
model over six months [5].
RESULTS:
Contact pressure distributions on the tibial plateau, were in good
agreement with those measured under the intact natural meniscus prior to
meniscetomy (Fig. #2). Peak and average pressures developed under the
implant were compared to those measured under the natural meniscus and
were found to be statistically indistinguishable (p≥0.05). Calculation of
peak/average pressure ratio (3.1±0.3) and contact area (658±135mm2) for the
implant were also statistically indistinguishable compared to those calculated
for the natural meniscus (2.7±0.5 and 642±96mm2, respectively). Outputs of
the FE model confirmed that internal strains/stresses within the device
components remain within the material’s allowed limits. Fatigue tests
demonstrated that both of the implant’s components, PCU and UHMWPE
fibers were not affected in the long term in respect to form, fiber-matrix
bonding and structure-function relationship. Specifically, no significant
dimensional changes were observed during the course of the test and pressure
distributions post 15-milion loading cycles remained similar to those
measured prior to the test. The evaluation of implant in a model adapted for
sheep showed no signs of wear or changes in structural or material properties.
Histological analysis showed relatively mild cartilage degeneration that was
dominated by loss of proteoglycan content and cartilage structure. However,
the total osteoarthritis score (Modified Mankin score) did not significantly
differ between the control and operated knees, and there were no differences
in the severity of degenerative changes between 3 and 6 months post-surgery.
Figure #2: Pressure maps
obtained for the natural and
optimal composite meniscal
implant design
DISCUSSION:
In the current study, we presented the development of a novel PCU
meniscal implant for the medial compartment of the knee, along with an
overview of essential tests. The main benefits claimed for this meniscal
implant are pain relief and preservation of meniscal functionality.
It was found that the implant’s closed discoid shape can provide a
larger, continuous bearing surface compared to the semi-lunar natural
meniscus. The implication of these being that (a) the implant is able to
reduce the overall cartilage load associated with meniscectomy by
effectively distributing joint loads, and (b) the implant completely
prevents contact between opposing cartilage surfaces. The results of
implantation in sheep can be considered exceptionally favorable, and
support the hypothesis that a PCU meniscal implant may counter the
occurrence of major degenerative cartilage changes following
meniscectomy. First implantations (Fig. #3) has shown that arthroscopic
implantation of the device is short and uncomplicated. Clinical followup of the device is underway.
Figure #3: Pre-operative (Left) and 24-month post implantation (Right)
MRI's of a male patient, 64 yrs., presented with medial meniscus deficiency
REFERENCES:
[1] Elsner JJ, Portnoy S, Guilak F, Shterling A, Linder-Ganz E. MRI-based
characterization of bone anatomy in the human knee for size-matching of a
medial meniscal implant. In Press, 2010
[2] Adams ME and Hukins DWL. The extracellular matrix of the meniscus.
New York, NY, Raven Press.1992
[3] Linder-Ganz E, Elsner JJ, Danino A, Guilak F, Shterling A. A novel
quantitative approach for evaluating contact mechanics of meniscal
replacements. J Biomech Eng. 132:024501, 2010
[4] Elsner JJ, Portnoy S, Zur G, Guilak F, Shterling A, Linder-Ganz E.
Design of a free-floating polycarbonate-urethane meniscal implant using
finite element modeling and experimental validation. J Biomech Eng. In
Press, 2010.
[5] Zur G, Linder-Ganz E, Elsner JJ et al. Chondroprotective effects of a
polycarbonate-urethane meniscal implant: histopathological results in a
sheep model. Knee Surg Sports Traumatol Arthrosc. In Press, 2010.
Poster No. 2117 • ORS 2011 Annual Meeting