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Consequences of ProDisc hyper-mobility.
Facet arthrosis and Pain
Long term facet arthrosis due to hyper-mobility.
Early Ball in Socket implants like Pro Disc inherently increase freedoms of movement without restraint. The fundamental function of the facets is to limit motion. It has been found that these implants will initiate or exacerbate the progression of facet Arthrosis.
Consequences of Prodisc hyperdynamics in a collapsed disc space with secondary facet arthrosis.
Abstract:
Total Disc Arthroplasty is a relatively new technology whose long-term effects are only extrapolated by short-term trials. One of the long-term concerns not well studied are the effects on the zygopophyseal joints. It is demonstrated here that the Prodisc-L exacerbates the stresses on the facets due to the forced shift of center of sagittal rotation from the anterior spinal canal to in the center of the lower keel. This is shown to increase the arc of rotation at the facets by a proportion equal to the change in radius distance about 2X. Furthermore, it is noted that the Prodisc inherently increases the freedoms of movement in all dimensions, excluding vertical axial elasticity. Given that the fundamental function of the facets is to limit motion, and that the facets are themselves limited in motion due to connective cartilage, it is presented that the hyperdynamics afforded by the Prodisc-L initiate or exacerbate the progression of facet Arthrosis.
Case Study:
To illustrate this effect, a case study is presented regarding a young (32 y/o) woman who pain and disability significantly increased after implantation of a Prodisc-L. After one year of complete disability, including inability to tolerate sitting or standing for more that a short interval, the patient's device was extracted laterally, and revised to a circumferential fusion. In review of the case, subject was diagnosed at 5 months Pre-op with DDD at one level, L4/5, all others normal. It was later discovered that the L4/5 space had collapsed to bone-on-bone, and that consequently the facets had progressed to Grade III arthrosis with significant hypertrophy. It was later found that this contributed to post-Prodisc segmental hyperlordosis and severe disabling pain as a result of the hyper-mobility afforded by the Prodisc's biomechanics.
Due to the results of Dr. Andre van Ooij et. al. on 75 revisions which reported 5.6/10 ODI with removal of the device, vs. 6.3/10 leaving in-situ, it was deemed prudent to remove the device. However, the Prodisc had induced an iatrogenic 2X natural intervertebral height, as compared to L3/4 and pre-DDD radiographs. Furthermore due to L4/5 vascular obstruction, the removal of the device laterally required significant osteotomy to free the lower keel. It was necessary to fill this 1.42cm space (PEEK packed with rh-BMP2) and secure by posterior rods/screws.
At one year, the patient's fusion exhibits psuedarthrosis with primarily the PEEK cage supporting a ring of solid osteogenesis. Thus, although diagnostic injections indicate complete pain relief might be attained by hardware removal, the patient is required to wait for a complete fusion. The following article investigates the etiology of this case.
Discussion
Many reports in the literature specify facet arthrosis as a leading cause of revision in disc arthroplasty. There have been reports of 32% increase in progression of facet arthrosis post-Prodisc, and studies indicating that even 1mm of execessive distraction of an invertebral disc space can lead to facet joint subluxation. Collapse of an intervertebral disc space is a known contraindication to artificial disc replacment. The collapse will often cause the posterior zygapophyseal joints to hypertrophy and fuse. Facet arthrosis results. Once fused, the facet joints lock the posterior. The level is no longer mobile and any motion at the level is likely to cause pain. Furthermore, If the Prodisc ( 11mm height from Prodisc Specs ), is significantly larger than the natural disc space (here: 8mm), then the vertebrae must be forced apart (distracted) anteriorly to insert the prothesis. Thus, inserting the device is tantamount to jamming a fulcrumed lever into the space.
It is self-evident that the Prodisc's center of rotation (sagittal) is in the lower keel, as this is the center of the ball-joint. Given that the natural 3-point vertebral segment (2 facets, disc) has a center-of-rotation in the posterior of the disc, it is clear that the Arc of Rotation induced on the facets is exacerbated. That is, the radius from the Keel-center to the facets is significantly larger than from the posterior-disc to facets.
Furthermore, the Prodisc allows 20 degrees sagittal rotation, compared to the 15 degrees seen Naturally. Doubling the Radius, as well as a 25% increase in rotation, leads to at least a doubled Arc-of-Rotation at the facets. Moreover, the Prodisc induces hyper-stresses on the facets due to its 2X of Natural lateral rotation, and it's 360 degrees vs. 10 degrees Natural 'axial rotation'.
Removal of a Prodisc at L4/5 has been called 'life-threatening in every case' - due to the scaring over the great veins which lie over the segment. Thus, the only recourse is to excavate the device laterally. Due to the Keels, this entails an Osteotomy (bone removal) of at least one of the vertebrae. Once one of the end-plates has been removed, it has been reported that the other can usually be pried out safely.
In this case, the Prodisc created an intervertebral space about 2X taller than Natural. This space had to be filled with a PEEK cage packed with rh-BMP2. It was not felt prudent to insert a shorter cage, restoring natural disc height and lordosis, as the cage could 'fall out' before osteogenesis secured it.
Secondary to the exacerbated intervertebral height, the process of osteogenesis is critically limited by the communication distance between the vertebrae. At one year, only the PEEK cage structure shows strong evidence of solid bone.
ref; http://www.prodisc-study.info/
Consequences_of_Prodisc_hyperdynamics_in_a_collapsed_
disc_space_with_secondary_facet_arthrosis.htm
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