Assessment of intraocular lens/capsular bag biomechanical interactions following cataract surgery in a human in vitro graded culture capsular bag model.

Intraocular lenses (IOLs) are implanted during cataract surgery. For optimum results, stable positioning of the IOL in the capsular bag is important. Wound-healing events following cataract surgery lead to modification of the capsular bag and secondary visual loss due to posterior capsule opacification. At present, it is unclear how these biological events can affect stability of the IOL within the capsular bag. In the present study, a human in vitro graded culture capsular bag model was the experimental system. Capsulorhexis and lens extraction performed on human donor eyes generated suspended capsular bags (5 match-paired experiments). Preparations were secured by pinning the ciliary body to a silicone ring and maintained in 6 mL of medium for 84 days using a graded culture system: days 1-3, 5% human serum and 10 ng/mL transforming growth factor ß (TGFß2); days 4-7, 2% human serum and 1 ng/mL TGFß2; days 8-14, 1% human serum and 0.1 ng/mL TGFß2; days 15-84, serum-free Eagle's minimum essential medium (EMEM). A CT LUCIA 611PY IOL was implanted in all preparations. Quantitative measures were determined from whole bag images captured weekly. Images were registered using FIJI and analysed in ImageJ to determine capsular bag area; distortion; angle of contact; haptic stability; capsulorhexis area; and a fusion footprint associated with connection between the anterior and posterior capsules. Cell coverage and light scatter were quantified at end-point. The transdifferentiation marker, α-SMA was assessed by immunocytochemistry. Immediately following surgery, distortion of the capsular bag was evident, such that a long axis is generated between haptics relative to the non-haptic regions (short axis). The angle of contact between the haptics and the peripheral bag appeared inversely correlated to capsular bag area. Growth on the peripheral posterior capsule was observed 1 week after surgery and beneath the IOL within 1 month. As coverage of the posterior capsule progressed this was associated with matrix contraction/wrinkles of both the central posterior capsule and peripheral capsular bag. Cells on the central posterior capsule expressed αSMA. Fusion footprints formed in non-haptic regions of the peripheral bag and progressively increased over the culture period. Within and at the edge of the fusion footprint, refractive structures resembling lens fibre cells and Elschnig's pearls were observed. Cell attachment to the IOL was limited. An impression in the posterior capsule associated with the CT LUCIA 611PY optic edge was evident; cell density was much greater peripheral to this indent. Wound-healing events following surgery reduced capsular bag area. This was associated with the long/short axis ratio and angle of contact increasing with time. In summary, we have developed a human capsular bag model that exhibits features of fibrotic and regenerative PCO. The model permits biomechanical information to be obtained that enables better understanding of IOL characteristics in a clinically relevant biological system. Throughout culture the CT LUCIA 611PY appeared stable in its position and capsular bag modifications did not change this. We propose that the CT LUCIA 611PY optic edge shows an enhanced barrier function, which is likely to provide better PCO management in patients.

Influence of trabecular bone quality and implantation direction on press-fit mechanics.

Achieving primary stability of uncemented press-fit prostheses in patients with poor quality bone can involve axial implantation forces large enough to cause bone fracture. Radial implantation eliminates intraoperative impaction forces and could prevent this damage. Platens of two commercial implant surfaces ("Beaded" and "Flaked") were implanted onto trabecular bone specimens of varying quality in a press-fit simulator. Samples were implanted with varying interference, either axially (shear) or radially (normal). Push-in and pull-out forces were measured to assess stability. Microstructural changes in the bone were determined from µCT analysis. For force-defined implantation analysis, push-in and pull-out forces both increased proportionally with increasing radial force, independent of implantation direction, bone quality or implant surface. For position-defined implantation analysis, pull-out forces were generally found to increase with interference and to be greater for radial than axial implantation direction, and to be lower for poor quality bone. Bone density increased locally at the tested interface due to implantation, in particular for the Beaded surface under axial implantation. If a safe radial stress can be determined for cortical bone in a particular patient, the associated implantation force, and pull-out force which represents primary stability, can be directly derived, regardless of implantation direction, bone quality or implant surface. Radial implantation delivers primary stability that is no worse than that for axial implantation and may eliminate potentially damaging impaction forces. Development of implant designs based on this principal might improve implant fixation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:224-233, 2017.

Friction coefficient and effective interference at the implant-bone interface.

Although the contact pressure increases during implantation of a wedge-shaped implant, friction coefficients tend to be measured under constant contact pressure, as endorsed in standard procedures. Abrasion and plastic deformation of the bone during implantation are rarely reported, although they define the effective interference, by reducing the nominal interference between implant and bone cavity. In this study radial forces were analysed during simulated implantation and explantation of angled porous and polished implant surfaces against trabecular bone specimens, to determine the corresponding friction coefficients. Permanent deformation was also analysed to determine the effective interference after implantation. For the most porous surface tested, the friction coefficient initially increased with increasing normal contact stress during implantation and then decreased at higher contact stresses. For a less porous surface, the friction coefficient increased continually with normal contact stress during implantation but did not reach the peak magnitude measured for the rougher surface. Friction coefficients for the polished surface were independent of normal contact stress and much lower than for the porous surfaces. Friction coefficients were slightly lower for pull-out than for push-in for the porous surfaces but not for the polished surface. The effective interference was as little as 30% of the nominal interference for the porous surfaces. The determined variation in friction coefficient with radial contact force, as well as the loss of interference during implantation will enable a more accurate representation of implant press-fitting for simulations.

The influence of bone damage on press-fit mechanics.

Press-fitting is used to anchor uncemented implants in bone. It relies in part on friction resistance to relative motion at the implant-bone interface to allow bone ingrowth and long-term stability. Frictional shear capacity is related to the interference fit of the implant and the roughness of its surface. It was hypothesised here that a rough implant could generate trabecular bone damage during implantation, which would reduce its stability. A device was constructed to simulate implantation by displacement of angled platens with varying surface finishes (polished, beaded and flaked) onto the surface of an embedded trabecular bone cube, to different nominal interferences. Push-in (implantation) and Pull-out forces were measured and micro-CT scans were made before and after testing to assess permanent bone deformation. Depth of permanent trabecular bone deformation ('damage'), Pull-out force and Radial force all increased with implantation displacement and with implantation force, for all surface roughnesses. The proposed hypothesis was rejected, since primary stability did not decrease with trabecular bone damage. In fact, Pull-out force linearly increased with push-in force, independently of trabecular bone damage or implant surface. This similar behaviour for the different surfaces might be explained by the compaction of bone into the surfaces during push-in so that Pull-out resistance is governed by bone-on-bone, rather than implant surface-on-bone friction. The data suggest that maximum stability is achieved for the maximum implantation force possible (regardless of trabecular bone damage or surface roughness), but this must be limited to prevent periprosthetic cortical bone fracture, patient damage and component malpositioning.