Does Vertebral Endplate Morphology Influence Outcomes in Lumbar Disc Arthroplasty? Part I: An Initial Assessment of a Novel Classification System of Lumbar Endplate Morphology

DISCUSSION

Many orthopaedic classifications have been developed that are based on the radiographic morphology of the structure of interest.^19–21 In disc arthroplasty this structure is the intervertebral disc space, and more specifically, the vertebral endplates. The morphology of vertebral endplates has been evaluated in the past to determine the correlation of the appearance of the endplates with disc herniation and the prevalence of low-back pain.²², ²³ Results have been mixed, but a common finding in these studies has been the extent of the variation in vertebral endplate morphology that exists in the population.

It is reasonable to consider the morphology of the vertebral endplate prior to TDR, as this is the structure on which the device is implanted. Morphologic differences in vertebral endplates do exist and have the potential to influence the positioning of TDR components; therefore, it may be important to classify these morphologies to assist in patient selection and surgical planning.

The classification of disease processes is quite useful. A reliable classification system allows physicians to properly characterize the problem, which eventually can guide treatment in a systematic manner. A classification system can also establish expected outcomes, which offers the ability to compare cases and treatments across groups of patients and physicians. The relatively new field of total disc arthroplasty would benefit greatly from a classification system that could assist in treatment guidance and offer some standard by which patients could be compared, despite the variety of implants currently used.

We have developed a novel classification system for patients undergoing lumbar total disc arthroplasty based on the preoperative lateral radiograph. This classification system was designed based on the observations of the two senior authors who have evaluated numerous patients for total disc arthroplasty. The criteria for this system have been defined in order to be as sensitive as possible, so as to identify all vertebral levels with morphologic anomalies. All patients classified as Type I have parallel endplates. Patients in Type II have endplates with a small posterior concavity followed by an endplate extension below the level of the anterior endplate. This posterior extension can inhibit the proper posterior positioning of the implant if not recognized and contoured appropriately (Figure 2). Type III endplates consist of either a superior or inferior endplate with greater than 10% concavity as compared to the total vertebral body height. Previous cadaveric studies have estimated the height of the normal lumbar vertebral body to be roughly 30 mm.²⁴ A 10% concavity would correlate with a 3 mm void in the central region of the endplate, which is the area in which the implant's central keel is placed to gain implant-bone fixation and ultimate bony ingrowth.

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Figure 2

A) Preoperative Type II endplate. (B) Postoperative Type II endplate with anterior positioning of superior L5 endplate due to posterior L5 hook anatomy.

The keel on the ProDisc-L is 6.5 mm in height. With these dimensions, a Type III endplate would lack approximately 50% surface area for endplate fixation and eventual bony ingrowth. Type IV endplates have an anterior convexity of greater than 20% vertebral body depth. The average vertebral body depth has been estimated to be approximately 35 mm.²⁴ Therefore, this type of endplate would offer approximately 28 mm of flat endplate to secure device fixation. The ProDisc-L is available in 2 sizes with depth measurements of 30 and 27 mm. Endplates with more than 20% anterior convexity may have an insufficient area on which to implant a prostheses (Figure 3). Type V endplates consist of any combination of Type II-IV morphologic changes.

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Figure 3

(A) Preoperative Type IV endplate. (B) Type IV endplate with convex S1 endplate and subsequent lack of endplate coverage.

We have shown this classification system to be reliable and reproducible, with kappa values between observers in the moderate range (κ = 0.34-0.51) and kappa values within endplates based solely on the measurements obtained in their review. This bias is a reasonable explanation for the difference in the kappa values between the observers. This explanation is further supported in reviewing the summated ratings from each observer (Table 2). Type II has been suggested by the classification developers to be quite important to identify because the posterior endplate extension, if not identified and addressed at the time of surgery, can lead to great difficulty in properly positioning the implant. Type II is radiographically similar to Type III.

Operative experience would likely bias an observer to identify more Type II endplates, as these would be more important to identify preoperatively so proper planning could be carried out. We noticed such bias as shown in Table 2. Observer 1 consistently identified more Type II endplates, whereas the less experienced observers consistently identified more Type III endplates.

Despite some variation in the interobserver reliability, we found a high degree of reliability in the intraobserver calculations for our classification, regardless of clinical experience. Kappa values were found to be 0.65 to 0.69, which is considered substantial agreement. The high level of agreement for this classification is likely due to the relative simplicity of the type definitions. This allows for more consistent analysis of the endplates individual observers in the substantial range (κ = 0.65-0.69). The results of our analysis are comparable to previously reported orthopaedic classifications that are commonly used. The Neer Classification for proximal humerus fractures has been widely used and analyzed. Sidor et al. reported intraobserver kappa values of 0.5-0.83 depending on the expertise and experience of the observer and interobserver values of 0.43-0.58.²¹

Spine classifications have shown similar results. Cummings et al. analyzed the King classification of scoliosis and showed intraobserver kappa values of 0.44 to 0.72, with an interobserver value of 0.44.²⁰ Finally, Grauer et al. recently offered a modification of the Anderson and D'Alonzo classification for odontoid fractures with intraobserver and interobserver kappa values of 0.41-0.6 and 0.48, respectively.¹⁹

We found a difference in the interobserver reliability with the experienced observer compared to the less experienced observers, but found no difference in the reliability between the 2 less experienced observers. This difference is likely due to bias from the more experienced observer. The experienced observer (who helped develop the classification) was likely influenced by his baseline knowledge in assessing patients for total disc arthroplasty and was less likely to grade endplates strictly according to measurements obtained in the review. The less experienced observers were more likely to grade the with objective standards, rather then relying on subjective assessment with each observation. A substantial amount of agreement was even seen for the most experienced surgeon, despite suggestions of subjective bias in his observations in the interobserver analysis. The intraobserver reliability is most important in classifications used for preoperative assessment of patients. A high intraobserver value allows surgeons to make consistent treatment decisions, which likely are more important for patient outcome than the interobserver reliability of a classification.

Like all classifications and their analysis, ours has its limitations. We did find evidence of observational bias by the most experienced physician in our study. Despite this limitation, the experienced physician had a high degree of intraobserver reliability in his measurements, suggesting this classification system is reliable even when exposed to subjective assessment. A potential limitation of our classification is the analysis of vertebral endplate morphology (a 3-dimensional structure) based on a single 2-dimensional radiograph. Although this is not ideal, we believe that if we could demonstrate the validity of our classification system using plain radiographs, then the use of additional, more advanced imaging such as sagittal CT reconstructions would only add to the accuracy and clinical usefulness of the system. Our classification system is simple and based on a standard radiograph that is routinely obtained in all chronic spinal conditions. Finally, our classification may be limited by its underlying definitions of the different types. We have attempted to define our classification based on findings that may impact clinical decision making or surgical planning.

We have based our measurements on reported averages of vertebral body measurements. Certainly variations exist in the dimensions of vertebral bodies, and as discussed above, we have tried to make our class definitions in such a way to include any subjects with potential clinically significant findings. This may predispose our classification to interpret radiographic findings as more clinically important then they may be. In classification systems used to assess patients for surgical procedures, we believe it is important to err on the side of increased sensitivity. This allows for the most thorough preoperative planning and ensures that potential clinically significant variations in anatomy are not missed.

We have introduced a classification for lumbar vertebral endplate morphology which may be useful in preoperative patient assessment prior to total disc arthroplasty. This classification appears to be reproducible and reliable across a range of clinical experience levels. Use of this classification system will be helpful in the preoperative planning for patients undergoing TDR, especially in preparing for the critically important endplate contouring aspect of the procedure. The information provided with this classification may eventually be useful in patient selection for total disc arthroplasty, as well as providing a rationale for implant selection, based on different base plate fixation designs and implant size options. Further prospective analysis is needed to validate this system's clinical usefulness and its potential to predict patient outcome.