The Cost of Joint Replacement: Comparing Two Approaches to Evaluating Costs of Total Hip and Knee Arthroplasty.

BACKGROUND: In an era of mandatory bundled payments for total joint replacement, accurate analysis of the cost of procedures is essential for orthopaedic surgeons and their institutions to maintain viable practices. The purpose of this study was to compare traditional accounting and time-driven activity-based costing (TDABC) methods for estimating the total costs of total hip and knee arthroplasty care cycles. METHODS: We calculated the overall costs of elective primary total hip and total knee replacement care cycles at our academic medical center using traditional and TDABC accounting methods. We compared the methods with respect to the overall costs of hip and knee replacement and the costs for each major cost category. RESULTS: The traditional accounting method resulted in higher cost estimates. The total cost per hip replacement was $22,076 (2014 USD) using traditional accounting and was $12,957 using TDABC. The total cost per knee replacement was $29,488 using traditional accounting and was $16,981 using TDABC. With respect to cost categories, estimates using traditional accounting were greater for hip and knee replacement, respectively, by $3,432 and $5,486 for personnel, by $3,398 and $3,664 for space and equipment, and by $2,289 and $3,357 for indirect costs. Implants and consumables were derived from the actual hospital purchase price; accordingly, both methods produced equivalent results. CONCLUSIONS: Substantial cost differences exist between accounting methods. The focus of TDABC only on resources used directly by the patient contrasts with the allocation of all operating costs, including all indirect costs and unused capacity, with traditional accounting. We expect that the true costs of hip and knee replacement care cycles are likely somewhere between estimates derived from traditional accounting methods and TDABC. TDABC offers patient-level granular cost information that better serves in the redesign of care pathways and may lead to more strategic resource-allocation decisions to optimize actual operating margins.

Comparison of Oxygen Consumption Rates of Nondegenerate and Degenerate Human Intervertebral Disc Cells.

STUDY DESIGN: In vitro measurements of the oxygen consumption rates (OCR) of human intervertebral disc (IVD) cells. OBJECTIVE: The aim of this study was to determine the differences in the OCR of nondegenerate and degenerate human annulus fibrosus (AF), nucleus pulposus (NP), and cartilage endplate (CEP) cells at different glucose concentrations. SUMMARY OF BACKGROUND DATA: The avascular nature of the IVD creates a delicate balance between rate of nutrient transport through the matrix and rate of disc cell consumption necessary to maintain tissue health. Previous studies have shown a dependence of OCR for animal (e.g., bovine and porcine) IVD cells on oxygen level and glucose concentration. However, the OCR of nondegenerate human IVD cells compared to degenerate human IVD cells at different glucose concentrations has not been investigated. METHODS: IVD cells were isolated from the AF, NP, and CEP regions of human cadaver spines and surgical samples. The changes in oxygen concentration were recorded when cells were sealed in a metabolic chamber. The OCR of cells was determined by curve fitting using the Michaelis-Menton equation. RESULTS: Under identical cell culture conditions, the OCR of degenerate human IVD cells was three to five times greater than that of nondegenerate human IVD cells. The degenerate IVD cells cultured in low-glucose medium (1 mmol/L) exhibited the highest OCR compared to degenerate cells cultured at higher glucose levels (i.e., 5 mmol/L, 25 mmol/L), whereas no significant differences in OCR were found among the nondegenerate IVD cells for all glucose levels. CONCLUSION: Considering the significantly higher OCR and unique response to glucose of degenerate human IVD cells, the degeneration of the IVD is associated with a cell phenotypic change related to OCR. The OCR of IVD cells reported in this study will be valuable for understanding human IVD cellular behavior and tissue nutrition in response to disc degeneration. LEVEL OF EVIDENCE: N/A.

Quantifying Baseline Fixed Charge Density in Healthy Human Cartilage Endplate: A Two-point Electrical Conductivity Method.

STUDY DESIGN: Regional measurements of fixed charge densities (FCDs) of healthy human cartilage endplate (CEP) using a two-point electrical conductivity approach. OBJECTIVE: The aim of this study was to determine the FCDs at four different regions (central, lateral, anterior, and posterior) of human CEP, and correlate the FCDs with tissue biochemical composition. SUMMARY OF BACKGROUND DATA: The CEP, a thin layer of hyaline cartilage on the cranial and caudal surfaces of the intervertebral disc, plays an irreplaceable role in maintaining the unique physiological mechano-electrochemical environment inside the disc. FCD, arising from the carboxyl and sulfate groups of the glycosaminoglycans (GAG) in the extracellular matrix of the disc, is a key regulator of the disc ionic and osmotic environment through physicochemical and electrokinetic effects. Although FCDs in the annulus fibrosus (AF) and nucleus pulposus (NP) have been reported, quantitative baseline FCD in healthy human CEP has not been reported. METHODS: CEP specimens were regionally isolated from human lumbar spines. FCD and ion diffusivity were concurrently investigated using a two-point electrical conductivity method. Biochemical assays were used to quantify regional GAG and water content. RESULTS: FCD in healthy human CEP was region-dependent, with FCD lowest in the lateral region (P = 0.044). Cross-region FCD was 30% to 60% smaller than FCD in NP, but similar to the AF and articular cartilage (AC). CEP FCD (average: 0.12 ±â€Š0.03 mEq/g wet tissue) was correlated with GAG content (average: 31.24 ±â€Š5.06 µg/mg wet tissue) (P = 0.005). In addition, the cross-region ion diffusivity in healthy CEP (2.97 ±â€Š1.00 × 10 cm/s) was much smaller than the AF and NP. CONCLUSION: Healthy human CEP acts as a biomechanical interface, distributing loads between the bony vertebral body and soft disc tissues and as a gateway impeding rapid solute diffusion through the disc. LEVEL OF EVIDENCE: N/A.

The region-dependent biomechanical and biochemical properties of bovine cartilaginous endplate.

Regional biomechanical and biochemical properties of bovine cartilaginous endplate (CEP) and its role in disc mechanics and nutrition were determined. The equilibrium aggregate modulus and hydraulic permeability between the central and lateral regions were examined by confined compression testing. Biochemical assays were conducted to quantify the amount of water, collagen, and glycosaminoglycan (GAG). The equilibrium aggregate modulus of the CEP in the central region (0.23 ± 0.15 MPa) was significantly lower than for the lateral region (0.83 ± 0. 26 MPa). No significant regional difference was found for the permeability of the CEP (central region: 0.13 ± 0.07×10(-15)m(4)/Ns and lateral region: 0.09 ± 0.03 × 10(-15)m(4)/Ns). CEPs were an average of 75.6% water by wet weight, 41.1% collagen, and 20.4% GAG by dry weight in the central region, as well as an average of 70.2% water by wet weight, 73.8% collagen, and 11.7% GAG by dry weight in the lateral region. Regional differences observed for the equilibrium aggregate modulus were likely due to the regional variation in biochemical composition. The lateral bovine endplate is much stiffer and may share a greater portion of the load. Compared with the nucleus pulposus (NP) and annulus fibrosus (AF), a smaller hydraulic permeability was found for the CEP in both the central and lateral regions, which could be due to its lower water content and higher collagen content. Our results suggest that the CEP may block rapid fluid exchange and solute convection, allow pressurization of the interstitial fluid, and play a significant role in nutrient supply in response to loading.

Effect of cartilage endplate on cell based disc regeneration: a finite element analysis.

This study examines the effects of cartilage endplate (CEP) calcification and the injection of intervertebral disc (IVD) cells on the nutrition distributions inside the human IVD under physiological loading conditions using multiphasic finite element modeling. The human disc was modeled as an inhomogeneous mixture consisting of a charged elastic solid, water, ions (Na+ and Cl-), and nutrient solute (oxygen, glucose and lactate) phases. The effect of the endplate calcification was simulated by a reduction of the tissue porosity (i.e., water volume faction) from 0.60 to 0.48. The effect of cell injection was simulated by increasing the cell density in the nucleus pulposus (NP) region by 50%, 100%, and 150%. Strain-dependent transport properties (e.g., hydraulic permeability and solute diffusivities) were considered to couple the solute transport and the mechanical loading. The simulation results showed that nutrient solute distribution inside the disc is maintained at a stable state during the day and night. The physiological diurnal cyclic loading does not change the nutrient environment in the human IVD. The cartilage endplate plays a significant role in the nutrient supply to human IVD. Calcification of the cartilage endplate significantly reduces the nutrient levels in human IVD. Therefore, in cell based therapy for IVD regeneration, the increased nutrient demand as a result of cell injection needs to be addressed. Excessive numbers of injected cells may cause further deterioration of the nutrient environment in the degenerated disc. This study is important for understanding the pathology of IVD degeneration and providing new insights into cell based therapies for low back pain.

Relationship between the length of time off work preoperatively and clinical outcome at 24-month follow-up in patients undergoing total disc replacement or fusion.

BACKGROUND CONTEXT: A recent study involving interbody fusion patients found that preoperative work status was significantly related to clinical outcome. In another study comparing the best and worst outcomes of total disc replacement, among a battery of variables analyzed, the only one that differentiated the best and worst outcome groups was the length of time off work before total disc replacement. PURPOSE: The purpose of this study was to determine if there was a relationship between the length of time off work and treatment outcome at 24-month follow-up. If such a relationship existed, a secondary study objective would be to determine if a duration of work could be identified beyond which would be associated with compromised clinical outcome. STUDY DESIGN AND SETTING: Data were collected prospectively from randomized clinical trials comparing total disc replacement with lumbar fusion conducted at a single site. PATIENT SAMPLE: A database of 232 patients enrolled in one of two Food and Drug Administration-regulated trials comparing total disc replacement with fusion for the treatment of symptomatic disc degeneration was the basis of the study group. Only patients who had reached 24-month follow-up were included. The 28 patients who were not employed by choice preoperatively were not included in the analysis. OUTCOME MEASURES: Primary outcome measures used were visual analog scales (VAS) assessing pain and Oswestry disability index. METHODS: The length of time off work before surgery was recorded in weeks. The mean percentage improvement between preoperative and 24-month follow-up scores were analyzed. RESULTS: There was a significant relationship between duration off work preoperatively and clinical outcome (p<.05). The length of time off work preoperatively was more strongly related to outcome than was surgery type, insurance type, job demand, or preoperative VAS and Oswestry scores. Further analysis of the data identified a window of approximately 0 to 13 weeks off work preoperatively that was related to significantly greater improvements on visual analog scales (VAS) and Oswestry scores after surgery compared with patients who were off work for more than 13 weeks preoperatively. Although the subgroup off work for more than 13 weeks improved significantly from their preoperative status, the improvement was not as great as in the subgroup off work for a shorter duration. CONCLUSIONS: The results of this study found that patients off work for a longer duration before surgery improved significantly postoperatively, but had less favorable clinical outcomes than patients off work for a lesser duration. This study suggests a window of approximately 13 weeks off work before surgery after which clinical improvement is reduced. Implications of this finding may be that once a patient becomes unable to work for an extended period, more rigorous psychological screening may be in order as well as perhaps engaging in more rigorous rehabilitation after surgery. Further investigation is needed to determine if the 13 weeks identified in this study is applicable to other populations.

Lumbar spinal arthroplasty: analysis of one center's twenty best and twenty worst clinical outcomes.

STUDY DESIGN: This is a retrospective analysis of data that were collected prospectively from 2 concurrent FDA IDE lumbar arthroplasty clinical trials performed at a single center. OBJECTIVES: The purpose of this study was to determine if factors differentiating those patients with the best and worst clinical outcomes from total disc arthroplasty (TDR) could be identified. SUMMARY OF BACKGROUND DATA: Overall the results of TDR have been favorable, including recent results from 2 FDA IDE trials conducted in the United States. However, as with any surgical procedure, there were some patients with extremely good outcomes, and some with poor outcomes. If factors differentiating these groups could be identified, this may help refine patient selection criteria and improve future results. METHODS: The databases of Charite and ProDisc patients at a single site were reviewed to identify patients who reached the 24-month follow-up period. A total of 203 patients, 63 who were implanted with the Charite prosthesis, and 140 who were implanted with the ProDisc prosthesis, were identified. The percentage change in the preoperative to postoperative VAS and Oswestry scores were used to identify the 10 best and 10 worst outcomes for each of the device types. Logistic regression analysis was conducted to determine which of a battery of demographic and clinical assessments were related to the best/worst group classification. RESULTS: Results of the regression analysis found that the only factor significantly related to clinical outcome was the length of time off work before surgery. None of the demographic variables, preoperative VAS or Oswestry scores or radiographic assessment of device placement, were significantly related to clinical outcome. Patients who were off work for shorter durations, or not at all, were more likely to be in the best-outcome group compared with patients who were off work for an extended period of time before surgery. CONCLUSION: This study suggests that among patients undergoing TDR, the length of time off work before surgery was related to outcome. No additional factors related to the best/worst classification were identified in the current study.

Comparison of results of total disc replacement in postdiscectomy patients versus patients with no previous lumbar surgery.

STUDY DESIGN: Data for this study were collected as part of a prospective Food and Drug Administration regulated IDE trial for the ProDisc total disc replacement. OBJECTIVE: The purpose of the study was to compare the results of total disc replacement (TDR) in patients with previous discectomy to patients who had no such previous surgery. SUMMARY OF BACKGROUND DATA: Discectomy is a well accepted surgical procedure for appropriately selected patients. However, removal of a portion of the disc may lead to further disc degeneration and collapse of the disc space. In the past, treatment options for such patients included a variety of fusion procedures, particularly ones incorporating an interbody graft to restore disc space height. Currently, TDR is being used to treat symptomatic disc degeneration. The results of this procedure have not been specifically investigated in patients with previous discectomy. METHODS: Data were drawn from 1 center participating in the IDE trial for ProDisc. Only patients reaching the 24-month follow-up period were included. Patients were divided into 2 subgroups: those who had previously undergone lumbar discectomy (N = 20) and those who had not (N = 67). Data for this study were collected prospectively before surgery, and at 6 weeks, 3, 6, 12, 18, and 24 months after surgery. The primary outcome measures used were visual analog scales (VAS) assessing pain, Oswestry questionnaire, postoperative patient satisfaction measured on a 0 to 10 VAS, and asking patients if they would have the same procedure again. The outcome measures were compared between the 2 subgroups. RESULTS: The results of this study found no differences between the 2 groups based on any of the 4 outcome measures used at any of the follow-up periods (all P > 0.10). Both groups improved significantly from the preoperative baseline on the VAS assessing pain and the Oswestry questionnaire. The mean satisfaction scores were greater than 7.5 at all follow-up periods and more than 80% of patients in both groups indicated that they would have the same surgery again. DISCUSSION: The results of this study indicate that the outcome of TDR is not compromised among patients witha history of previous discectomy. This finding supports TDR as a viable treatment for patients with symptomatic disc degeneration arising from a previously operated lumbar disc.

Two- to 3-Year Follow-Up of ProDisc-L: Results From a Prospective Randomized Trial of Arthroplasty Versus Fusion.

BACKGROUND: Although total disc replacement has been performed for years outside the United States, relatively little available data address clinical outcomes, particularly data from prospective studies. We report the 24- to 36-month follow-up of one center's experience with the ProDisc-L artificial disc as part of a prospective, randomized trial comparing total disc arthroplasty to combined anterior-posterior lumbar fusion. METHODS: The study involved clinical results for 157 patients from a single center enrolled in the US Food and Drug Administration-regulated trial comparing ProDisc-L to fusion. Only patients who had reached a minimum 24-month follow-up were included in the study. Patients were randomized to receive total disc replacement or circumferential fusion at 1 or 2 lumbar disc levels from L3 to S1, with specific inclusion and exclusion criteria. Data were collected preoperatively and at 6 weeks to 36 months postoperatively. The primary clinical outcome measures were Visual Analog Scale (VAS) scores to assess pain and Oswestry Disability Index (ODI) scores to measure function. RESULTS: The VAS and ODI scores in both treatment groups improved significantly as early as the 6-week followup visit and remained significantly improved throughout the 36-month follow-up period. Although a tendency was observed for the ProDisc-L scores to indicate more favorable outcome, the differences were not statistically significant. The proportion of patients who would have the same procedure again was greater in the total disc replacement group at all follow-up intervals, and significantly greater at the 6- month, 12-month, 24-month, and 36-month follow-up visits. CONCLUSIONS: The results of this study indicate that the total disc replacement with ProDisc-L produces improvements in pain and function that are at least as good as those provided by circumferential fusion. During the long-term follow-up of 24 and 36 months, outcomes did not become less favorable compared with the early outcomes. CLINICAL RELEVANCE: We found that results of total disc replacement were at least as good as those achieved with combined instrumented anterior-posterior fusion for the treatment of painful disc degeneration. Favorable results were maintained during 24- and 36-month follow-up.

Lumbar spine arthroplasty: early results using the ProDisc II: a prospective randomized trial of arthroplasty versus fusion.

This study represents the first 39 patients with at least 6-month follow-up enrolled in a prospective randomized Food and Drug Administration study evaluating the safety and efficacy of the ProDisc II versus the control, a 360 degrees lumbar spinal fusion. Data were collected preoperatively and at 6 weeks, 3 months, and 6 months postoperatively. Visual Analog Scale (VAS), Oswestry Low Back Pain Disability Questionnaire (ODQ), and patient satisfaction rates were evaluated at these intervals, as well as range of motion, return to work, and recreational and ambulatory status. There were 28 ProDisc patients and 11 who underwent fusion. Six patients had two-level surgery. Estimated blood loss (ProDisc = 69 mL versus fusion = 175 mL) and operative time (ProDisc = 75 minutes versus fusion = 219 minutes) were significantly different (P < 0.01). Hospital stays were shorter (ProDisc = 2.1 days versus fusion = 3.5 days [P < 0.01]) for ProDisc patients. There was a significantly greater reduction in the ODQ scores at 3 months in the ProDisc group compared with the fusion group (P < 0.05). No difference was noted in VAS. A trend was identified at 6 months in patient satisfaction rates favoring ProDisc versus fusion (P = 0.08), and motion was significantly improved in ProDisc patients compared with the fusion group (P = 0.02). Ambulatory status as well as recreational activity improved faster in the ProDisc group. The data suggest that total disc arthroplasty may be an attractive option as opposed to lumbar fusion for the surgical treatment of disabling mechanical low back pain secondary to lumbar disc disease.