The most important findings that our study established include a reduction in bonding strength in high viscosity cement verses low viscosity cement (P < 0.001), an overall trend between time to application and pull-off strength, and a reduction in bonding strength on coated verses uncoated surfaces (P = 0.037). Although multifactorial failures of this variety are often difficult to predict and even more difficult to ascertain cement adhesion deficiencies, it appears that at least two, if not three, combinations may be in play for suboptimal fixation. Clearly the cement viscosity link to the surface technology is vital.
Secondly, this study suggests there to be an overall inferior fixation of cement to a coated surface. What is interesting, however, is the individual trend for particular cement types with respect to coating status. The finding that each cement type responds differently to coating status, as seen in Fig. 3, that implores a further exploration of the current implant-cement combinations being utilized.
Finally, this study shows that there is an unrecognized variation in implant adhesive properties. Our conclusion that low viscosity cement is statistically superior to high viscosity cement in regards to pull-off strength is consistent with prior findings published in the previous few years of high viscosity cement having a higher susceptibility for aseptic loosening of the tibial component than other cements [3, 6, 11, 12, 14]. Ultimately, coated implants used with high viscosity cement may have inferior adhesion strength, which could be detrimental to implant durability. Despite this, the integrity of the coated surface has been suspect for some time. Fixation of cement is not a covalent atom-to-atom link, but instead a dipole-dipole interaction that creates polarization effects on a mechanical roughened substrate. As this is exposed to water, bonding strength decreases, resulting in lower yield strength from hydrolyzation [7]. Secondly, crack propagation in cement may also weaken the cement at the bond junction. Pre-treatment with SiOx has been reported to greatly enhance bond strength, creating Van der Waals ionic bonds with the silicon monoxide layer, stabilizing the hydrolytic debonding while lessening surface crack propagation [20, 22]. However, the pre-treatment is not present on this implant or others to our knowledge.
Strengths and limitations
Weakness and limits exist in any study, as ours is not immune in this particular scenario. The most profound is the extremely narrow “workable phase” where extended time caused massive weakness in cement strength. In this series, samples were never tested beyond 5 minutes to lesson confounding variables of cement adhesion. Also, a number of dowel plugs extruded cement despite attempts to clean around the base, which may affect adhesion, as the surface area would be larger than the 4.5 mm cone. We sought to limit this by making an area normalization adjustment. Extruded cement represents a confounding variable inherent in any type of force study, of which this study is no exception.
We did not test any other makers of implants, but merely looked at the surface coating as a standard amongst the brand name utilized in this case. While this does not condemn surface technology modifications, it by no means excuses the importance at looking at what that may do in terms of the fixation long-term. There have been some questions regarding adhesion durability of cement fixation in vivo. The longer the implant remained in the patient, the more likely cement would loosen on tibial trays [8, 24]. This was confirmed in vitro studies, where after 60 days of incubation, strength of cement adhesion on ceramic coated cobalt chrome was completely unbonded [4, 26]. Comparatively, given similar surface there is no published data on differences in chrome cobalt and titanium. It is of note that the surface roughness of the coated implant is identical to the chrome cobalt. One report of pull out strength noted similar strength. However, a trend for inferior cement adhesion to ceramic was noted in the published data. This study was flawed due to the complexity of failure modes of sheer and bonding pull off failure that were not isolated [9]. It also should raise some concern in the orthopedic community that there are in fact differences in adhesion properties between certain cement brands and may extend to the technique the surgeon utilizes in terms of early verses late curing phase. The high viscosity cement is by far easier to work with than the low viscosity; however, if one is compelled to use coating systems, it may well serve the surgeon to ensure these surfaces have been tested to failure and not merely tested on what federal guidelines, ASTM, and ISO standards suggest are appropriate. It is also apparent that while we did not test cement in the late phase of curing but instead in the early “sticky” phase, one can only imagine what the results of our testing, let alone the clinically implications, would have been using cement during the late phases of curing. It is also important that our study, as are the majority of those cited above, is an in vitro study.
Additional sources of potential limitations include the size of the data set and the distribution of the data set. The size was relatively small, with a total of 153 implant-cement combinations tested. Ideally, the findings from our data would prompt a more extensive and larger exploration of the limitations of current cement, which would reduce any confounding variables that might be present in our data set. We found that our data were positively skewed (skewness = 1.7); however, this is unlikely to negate our findings and would be limited as well by future testing with more data points.