Ultra-High-Molecular-Weight Polyethylene Merlon Shape: Novel Fixation of Artificial Bone for Cranioplasty

BACKGROUND

Cranioplasty is a surgical procedure widely performed for repairing cranial defects caused by external decompression surgery for cerebrovascular disease or traumatic brain injury. We devised a new cranioplasty method using artificial bone made up of ultra-high molecular-weight polyethylene, with serrated wings on the edge. We named this newly designed artificial bone as Merlon shape.

OBJECTIVE

To describe our initial experience with the Merlon shape and evaluate its usefulness and safety in cranioplasty.

METHODS

The serrated wings of the Merlon shape were preoperatively designed for solid fixation and improving cosmetic results by reducing the thickness of the artificial bone. We evaluated 25 patients who underwent cranioplasty with the Merlon shape between December 2018 and December 2021. The causes of bone defects in these patients (male: 9, female: 16; median age: 62 years) were subarachnoid hemorrhage (n = 14), cerebral infarction (n = 8), and traumatic brain injury (n = 3).

RESULTS

There were no postoperative adverse events such as infection, bone resorption, implant exposure, or graft sinking in 24 patients during an average follow-up period of 19 months. One patient experienced acute epidural hemorrhage and required reoperation.

CONCLUSION

This is the first report on the use of the ultra-high molecular-weight polyethylene Merlon shape. Our initial 4-year case series showed good outcomes with this method.

Polyethylene wear in retrieved reverse total shoulder components

BACKGROUND

Reverse total shoulder arthroplasty has been used to treat rotator cuff tear arthropathy and proximal humerus fractures, as well as for failed conventional total shoulder prostheses. It has been suggested that polyethylene wear is potentially higher in reverse shoulder replacements than in conventional shoulder replacements. The modes and degree of polyethylene wear have not been completely elucidated. The purpose of this study was to evaluate polyethylene wear patterns in 7 specimens retrieved at revision arthroplasty and identify factors that may be associated with increased wear.

METHODS

Reverse total shoulder components were retrieved from 7 patients during revision arthroplasty for loosening and/or pain. Preoperative glenoid tilt and placement and scapular notching were evaluated by use of preoperative radiographs. Polyethylene wear was evaluated via micro-computed tomography and optical microscopy.

RESULTS

Wear on the rim of the polyethylene humeral cup was identified on all retrieved components. The extent of rim wear varied from a penetration depth of 0.1 to 4.7 mm. We could not show a correlation between scapular notching and rim wear. However, rim wear was more extensive when the inferior screw had made contact with the liner. Metal-on-metal wear between the humeral component and the inferior screw of 1 component was also observed. Wear of the intended bearing surface was minimal.

DISCUSSION

Rim damage was the predominant cause of polyethylene wear in our retrieved specimens. Direct contact between the humeral component and inferior metaglene screws is concerning because this could lead to accelerated ultra-highmolecular weight polyethylene wear and also induce mechanical loosening of the glenoid component.

The Latest Lessons Learned from Retrieval Analyses of Ultra-High Molecular Weight Polyethylene, Metal-on-Metal, and Alternative Bearing Total Disc Replacements

Knowledge regarding the in vivo performance and periposthetic tissue response of cervical and lumbar total disc replacements (TDRs) continues to expand. This review addresses the following four main questions: 1) What are the latest lessons learned from polyethylene in large joints and how are they relevant to current TDRs? 2) What are the latest lessons learned regarding adverse local tissue reactions from metal-on-metal, CoCr bearings in large joints and how are they relevant to current TDRs? 3) What advancements have been made in understanding the in vivo performance of alternative biomaterials, such as stainless steel and polycarbonate urethane, for TDRs in the past five years? 4) How has retrieval analysis of all these various artificial disc bearing technologies advanced the state of the art in preclinical testing of TDRs? The study of explanted artificial discs and their associated tissues can help inform bearing selection as well as the design of future generations of disc arthroplasty. Analyzing retrieved artificial discs is also essential for validating preclinical test methods.

Crack initiation in retrieved cross-linked highly cross-linked ultrahigh-molecular-weight polyethylene acetabular liners: an investigation of 9 cases

Nine cross-linked highly cross-linked ultrahigh-molecular weight polyethylene acetabular liners were retrieved at revision surgery. Eight of the liners were fully intact and functional at retrieval. Six cases contained shallow initiated cracks at the root of rim notches; 1 crack had propagated several millimeters. Optical and electron microscopic inspection of the crack surfaces revealed clam shell markings, which are characteristic of fatigue crack initiation. Crack initiation at notches has been identified in reports of catastrophic cross-linked liner failures, with crack initiation sites exhibiting similar morphology and clam shell markings. Thus, we believe that the shallow cracks identified in this case series are precursors to catastrophic rim fracture. The results of this study recommend further investigations to clarify the etiology and prevalence of crack initiation in cross-linked acetabular liners.

Reasons for revision of first-generation highly cross-linked polyethylenes

Over a 10-year period, we prospectively evaluated the reasons for revision of contemporary and highly cross-linked polyethylene formulations in amulticenter retrieval program. Two hundred twelve consecutive retrievals were classified as conventional gamma inert sterilized (n = 37), annealed (Cross fire,[Stryker Orthopedics, Mahwah, NJ] n = 72), or remelted (Longevity [Zimmer ,Warsaw, Ind], XLPE[Smith and Nephew, Memphis, Tenn], Durasul [Zimmer,Warsaw, Ind] n = 103) liners. The most frequent reasons for revision were loosening (35%), instability(28%), and infection (21%) and were not related to polyethylene formulation (P = .17). Annealed and remelted liners had comparable linear penetration rates(0.03 and 0.04 mm/y, respectively, on average), and these were significantly lower than the rate in conventional retrievals (0.11 mm/y, P ≤ .0005). This retrieval study including first-generation highly cross linked liners demonstrated lower wear than conventional polyethylene. Although loosening remained as the most prevalent reason for revision, we could not demonstrate a relationship between wear and loosening.The long-term clinical performance of first-generation highly cross-linked liners remains promising based on the midterm outcomes of the components documented in this study [corrected].

Evaluation of J-initiation fracture toughness of ultra high molecular weight polyethylene used in total joint replacements

Fracture of ultra high molecular weight polyethylene (UHMWPE) total joint replacement components is a clinical concern. Thus, it is important to characterize the fracture resistance of UHMWPE. To determine J-initiation fracture toughness (J(Q)) for metals and metallic alloys, ASTM E1820 recommends a procedure based on an empirical crack blunting line. This approach has been found to overestimate the initiation toughness of tough polymers like UHMWPE. Therefore, in this study, a novel experimental approach based on crack tip opening displacement (CTOD) was utilized to evaluate J(Q) of UHMWPE materials. J-initiation fracture toughness was experimentally measured in ambient air and a physiologically-relevant 37°C PBS environment for three different formulations of UHMWPE and compared to the blunting line approach. The CTOD method was found to provide J(Q) values comparable to the blunting line approach for the UHMWPE materials and environments examined in this study. The CTOD method used in this study is based on experimental observation and, thus, does not rely on an empirical relationship or fracture surface measurements. Therefore, determining J(Q) using the experimentally based CTOD method proposed in this study may be a more reliable approach for UHMWPE and other tough polymers than the blunting line approach.

What is the correlation of in vivo wear and damage patterns with in vitro TDR motion response?

STUDY DESIGN

This study combined the evaluation of retrieved total disc replacements (TDRs) with a biomechanical study using human lumbar spines. Thirty-eight CHARITE TDRs were retrieved from 32 patients after 7.3 years average implantation. All implants were removed because of intractable back pain and/or facet degeneration. In parallel, 20 new implants were evaluated at L4-L5 and L5-S1 in an in vitro lumbar spine model.

OBJECTIVE

The purpose of this study was to correlate wear and damage patterns in retrieved TDRs with motion patterns observed in an in vitro lumbar spine model. We also sought to determine whether one-sided wear and motion patterns were associated with greater in vivo wear.

SUMMARY OF BACKGROUND DATA

The comparison of polyethylene wear in TDRs after long-term implantation to those tested using an in vitro model had not yet been investigated.

METHODS

The wear patterns of each retrieved PE core was analyzed at the rim and dome. Thirty-five cores were further analyzed using MicroCT to determine the penetration symmetry. For the in vitro study the implants were tested under physiologic loads using a validated cadaveric model. Motion patterns of the in vitro-tested implants were tracked using sequential video-fluoroscopy.

RESULTS

Fifteen of 35 retrieved cores (43%) displayed one-sided wear patterns. Significant correlations were observed between implantation time and penetration and penetration rate. In the in vitro study, there was evidence of motion at both articulations, motion at both articulation but predominantly at the top articulation, and solelyat the top articulation. Core entrapment and pinching was observed and associated with visual evidence of core bending or deformation.

CONCLUSION

This is the first study to directly compare the long-term PE wear and damage mechanisms in TDR retrievals with the motion patterns generated by a validated in vitro cadaveric testing model. The retrievals exhibited wear patterns consistent with the in vitro testing.