Defining the Minimal Clinically Important Difference and Patient Acceptable Symptom State for Microfracture of the Knee: A Psychometric Analysis at Short-term Follow-up

Abstract

Background:

Several studies have investigated failure rates and magnitude of improvement in patient-reported outcome measures after microfracture surgery for focal chondral defects of the knee; however; what constitutes clinically significant improvement in this patient population is poorly understood.

Purpose:

To (1) establish the minimal clinically important difference (MCID) and patient acceptable symptom state (PASS) thresholds for microfracture surgery including the time-dependent nature of these thresholds and (2) identify predictors of achieving the MCID and PASS in patients specifically undergoing microfracture of the knee.

Study Design:

Case series; Level of evidence, 4.

Methods:

A secure institutional cartilage preservation repository was queried for all patients who underwent microfracture between 2004 and 2017. The distribution method was used to calculate MCID thresholds for the International Knee Documentation Committee (IKDC) score and the Knee injury and Osteoarthritis Outcome Score (KOOS), whereas an anchor-based method was used for the PASS. Multivariate logistic regressions were constructed to determine predictors of achieving the MCID and PASS.

Results:

A total of 206 patients with a mean ± SD age of 33.7 ± 13.2 years and body mass index of 26.9 ± 5.3 kg/m² were included. All thresholds for the MCID and PASS increased over time except for the MCID thresholds for the KOOS Sports and Symptoms subscales. The proportion of patients who achieved the MCID (6 months, 78.4%; 12 months, 83.9%; 24 months, 88.6%) and PASS (6 months, 67.7%; 12 months, 79.2%; 24 months, 76.1%) generally increased over time. Older age and larger lesion size were negative independent predictors of MCID achievement. Older age was also a negative predictor of the PASS, whereas male sex and higher preoperative KOOS Symptoms and Pain scores were positive independent predictors of the PASS.

Conclusion:

The MCID and PASS thresholds for the IKDC and KOOS in patients undergoing microfracture of the knee are dynamic, with an increasing number of patients achieving the MCID over time. The percentage achieving the PASS increased between 6 and 12 months and then declined slightly at 24 months. Independent predictors of achieving the MCID were lesion size and age at surgery, whereas predictors of achieving the PASS included lesion size, male sex, and greater preoperative KOOS Symptoms and Pain scores.

Keywords

microfracture clinical outcomes minimal clinically important difference patient acceptable symptom state psychometric

Microfracture surgery of the knee remains the most common reparative procedure for small focal chondral defects due to its relatively low cost, low technical demand, and low need for additional equipment.^11,12 Within the field of cartilage repair and regeneration, recent trends have included the development of alternative knee preservation procedures along with an emphasis on clinically significant improvement.¹⁰ The minimal clinically important difference (MCID) and patient acceptable symptom state (PASS) are 2 such clinically meaningful measures that are increasingly reported in knee preservation surgery. The MCID is defined as the smallest difference in a domain of interest that patients perceive as clinically significant and detectable,^5,7 whereas the PASS is an intermediate outcome state in which a patient finds his or her function to be satisfactory.^1,8

To better understand the clinical effect of the microfracture procedure, it is important to define clinically meaningful outcome measures such as the MCID and PASS to distinguish between statistically significant and clinically significant outcomes. Although clinically significant outcome thresholds have been established for other cartilage preservation procedures of the knee, such as osteochondral allograft transfer,¹⁷ fresh osteochondral allograft transplant,²⁴ and autologous chondrocyte implant,¹⁶ such thresholds have not been established specifically for microfracture.

Given that these psychometric measures have yet to be defined for microfracture specifically, the purposes of the current study were to (1) establish the MCID and PASS thresholds for microfracture surgery including the time-dependent nature of these thresholds and (2) identify predictors of achieving the MCID and PASS in this specific cohort. It was hypothesized that the MCID and PASS would change over time and that several pre- and intraoperative patient variables would be predictive of achieving the MCID and PASS.

Methods

Patient Selection

The current study received institutional board approval for prospective collection of patient information and outcomes in a secure clinical repository as well as retrospective query and analysis. All patients who were seen at our institution for symptomatic knee pain and subsequently underwent a primary microfracture procedure by a single, fellowship-trained surgeon (B.J.C.) between January 2004 and January 2017 were queried (n = 521). Failure of nonoperative management (nonsteroidal anti-inflammatory medications, physical therapy, and corticosteroid injections) and a minimum of 2-year follow-up were required for inclusion. Exclusion criteria consisted of patients undergoing revision microfracture procedures, patients undergoing microfracture after a previous cartilage-preserving procedure (osteochondral allograft or autograft transplant, autologous chondrocyte implant, or novel biologic scaffolding), patients with a history of traumatic ipsilateral knee injuries, patients who underwent microfracture of the femoral notch, and patients who did not have documented preoperative outcome measures as this would prevent calculation of clinically significant outcome improvement. A total of 27 patients did not have minimum 2-year follow-up, and 288 patients did not have documented preoperative outcome measures, leaving 207 patients for inclusion in the final analysis. Pre- and intraoperative variables including sex, smoking status, body mass index (BMI), age, concomitant procedures, and number and size of cartilage defects (cm²) were subsequently extracted for all patients. This list of variables was considered and later integrated into multivariate analyses, as they have been shown to influence outcomes in previous literature regarding microfracture of the knee.^3,20,25

Quantification of Clinically Significant Outcome Thresholds

To quantify the clinical significance of meaningful outcome achievement, the MCID and PASS were calculated for International Knee Documentation Committee (IKDC) subjective knee form and the following Knee injury and Osteoarthritis Outcome Score (KOOS) subscales: (1) Activities of Daily Living (ADL), (2) Pain, (3) Quality of Life (QOL), (4) Sports, and (5) Symptoms. The MCID was calculated using the distribution method derived from the value equal to half of the standard deviation for each knee outcome tool for the overall cohort.^7,15 MCID was independently calculated and reported at 6-month, 12-month, and 24-month follow-up points. Patients were classified as achieving the MCID if the MCID was achieved on any of the included outcome measures.

The PASS was calculated through use of an anchor-based method.^1,2 At 6-month, 12-month, and 24-month time points, patients were asked the following anchor question: “Taking into account all the activities you have during your daily life, your level of pain, and also your functional impairment, do you consider that your current state is satisfactory?” The anchor question allows for the determination of an absolute postoperative score that is associated with an “intermediate amount of improvement.” We then identified PASS threshold values by using a receiver operating characteristic (ROC) curve analysis and subsequently calculating the Youden index, as this criterion optimizes the sensitivity and specificity of the threshold value (Figure 1). The area under the curve (AUC) of the ROC analysis was used to determine the significance of the identified PASS values. The strength of association was considered acceptable if the AUC exceeded 0.7 and excellent if greater than 0.8.¹⁴ Patients were classified as achieving the PASS if the PASS was achieved on any of the included outcome measures.

Statistical Analysis

Statistical analyses were performed with Microsoft Excel (Microsoft Corporation) and SPSS statistical software (version 23.0; IBM SPSS Statistics for Windows). Before analysis, all data were screened to determine whether they met parametric statistical assumptions. Continuous variables are reported as means and standard deviations, whereas categorical variables are presented as relative frequencies and percentages. A repeated-measures 1-way analysis of variance with post hoc Greenhouse-Geisser and Bonferroni corrections was conducted to determine statistically significant changes in patient-reported outcome scores across all postoperative time points. Pearson and Spearman rank analyses were performed to identify correlations between pre- and intraoperative variables and the MCID and PASS to identify final variables to incorporate into the final logistic regression models. Variables demonstrating statistically significant correlations were incorporated into multivariate logistic regression models to determine predictors of achieving clinically significant outcome improvement for the microfracture-specific cohort. Statistical significance was set at an α≤ .05.

Results

Patient Characteristics

The study included a total of 206 patients with a mean ± SD age of 33.7 ± 13.2 years and BMI of 26.9 ± 5.3 kg/m². Of these patients, 55.7% were male. A total of 18 (8.7%) patients indicated that they were smokers at the time of surgery. The mean follow-up for the cohort was 7.6 ± 4.2 years. Intraoperative information regarding lesion size, location, and number is provided in Table 1.

Table 1

Intraoperative Lesion Characteristics^a

	Value
Lesion size, mean ± SD, cm	1.4 ± 1.1
Lesion size >2 cm²	74 (35.9)
Lesion location
LFC	28 (13.6)
LFC + LTP	1 (0.5)
LFC + trochlea	1 (0.5)
MFC	68 (33.0)
MFC + LFC	2 (1.0)
MFC + trochlea	4 (1.9)
MFC + CTP	1 (0.5)
MFC + MTP	2 (1.0)
MFC + patella	1 (0.5)
Patella	17 (8.3)
Trochlea	43 (20.9)
MTP	6 (2.9)
LTP	17 (8.3)
CTP	2 (1.0)
Isolated lesion	193 (93.7)

Values are expressed as n (%) unless otherwise indicated. CTP, central tibial plateau; LFC, lateral femoral condyle; LTP, lateral tibial plateau; MFC, medial femoral condyle; MTP, medial tibial plateau.

Functional Outcomes Analysis

Statistically significant mean improvements in postoperative outcome scores were observed at the P < .05 level for all time points when compared with baseline preoperative scores (Table 2, Figure 2).

Table 2

Comparison of Outcome Scores at 6, 12, and 24 Months Versus Preoperative Outcome Scores^a

	Preoperative	6 Months	12 Months	24 Months	P Value
IKDC	43.2 ± 15.8	50.8 ± 17.5	58.7 ± 19.7	63.4 ± 22.1	.003
KOOS ADL	67.4 ± 24.0	79.3 ± 21.9	83.1 ± 21.2	84.7 ± 23.6	.002
KOOS Pain	56.1 ± 17.5	66.1 ± 18.6	74.3 ± 18.0	69.0 ± 20.7	.008
KOOS QOL	24.2 ± 17.7	34.3 ± 22.7	43.7 ± 24.6	44.1 ± 23.5	.012
KOOS Sports	34.5 ± 24.3	40.7 ± 25.5	52.7 ± 32.4	51.3 ± 30.0	.024
KOOS Symptom	62.1 ± 17.8	68.4 ± 22.4	72.3 ± 18.1	70.1 ± 19.7	.16

Values are presented as mean ± SD. Bolded P values indicate statistically significant changes over the study period. ADL, Activities of Daily Living; IKDC, International Knee Documentation Committee score; KOOS, Knee injury and Osteoarthritis Outcome Score; QOL, Quality of Life.

Psychometric Analysis

The MCID and PASS values for the IKDC subjective knee form in addition to the KOOS subscales are reported in Table 3. All MCID thresholds increased over time, with the exception of the KOOS Sports and Symptoms subscales, which demonstrated a minor overall decrease at the 24-month time point. Similarly, for the PASS, all thresholds increased over time.

Table 3

Microfracture-Specific MCID and PASS Thresholds^a

Outcome Measure	6 Months	12 Months	24 Months
MCID
IKDC	10.1	10.3	16.7
KOOS ADL	8.7	9.2	11.2
KOOS Pain	8.9	9.1	10.5
KOOS QOL	12.9	13.4	14.3
KOOS Sports	15.2	14.9	15.1
KOOS Symptoms	10.9	8.3	10.2
PASS
IKDC	51.2	53.4	68.7
KOOS ADL	88.2	89.0	94.1
KOOS Pain	70.8	72.2	80.6
KOOS QOL	46.9	46.9	53.1
KOOS Sports	37.5	42.5	62.5
KOOS Symptoms	76.8	77.4	82.1

ADL, Activities of Daily Living; IKDC, International Knee Documentation Committee score; KOOS, Knee injury and Osteoarthritis Outcome Score; MCID, minimal clinically important difference; PASS, patient acceptable symptom state; QOL, Quality of Life.

The percentage of patients who achieved clinically significant outcome was reported at each postoperative timepoint (Figure 3). For the MCID, the proportion of patients achieving a clinically significant outcome increased with time from the index microfracture procedure (6 months, 78.4%; 12 months, 83.9%; 24 months, 88.6%). However, for the PASS, more patients reported achievement at 12 months postoperatively with a lower proportion by 24 months (6 months, 67.7%; 12 months, 79.2%; 24 months, 76.1%).

Predictors of Clinically Significant Outcome Improvement

Variables that demonstrated a statistically significant relationship in the Pearson and Spearman rank correlation analyses were incorporated into the final multivariate logistic regression models to determine predictors of the MCID and PASS at 6, 12, and 24 months after microfracture (Table 4). These variables included age, BMI, sex, smoking status, lesion size, lesion size greater than 2 cm², and KOOS subscale scores. Larger lesion size (odds ratio [OR], 0.41; P = .004) and lesions greater than 2 cm² (OR, 0.89; P = .048) were both independently associated with a lower likelihood of achieving the MCID for any outcome measure at 6 months postoperatively. Increasing age (OR, 0.61; P = .036) was independently associated with a lower likelihood of achieving the MCID for any outcome measure at 24 months postoperatively. There were no independent predictors of the MCID at 12 months postoperatively.

Table 4

Multivariate Logistic Regression Models for Predictive Value of Pre- and Intraoperative Variables on MCID and PASS^a

		95% CI
Predictive Variables	Odds Ratio	Lower	Upper	P Value
MCID: 6 months
Lesion size, cm²	0.41	0.22	0.75	.004
Lesion size >2 cm	0.89	0.71	0.98	.048
MCID: 24 months
Age	0.61	0.33	0.94	.036
PASS: 6 months
Male sex	2.02	1.67	8.2	.016
Preoperative KOOS Symptoms	1.03	1.0	1.71	.049
Preoperative KOOS Pain	1.08	1.01	1.20	.040
PASS: 24 months
Male sex	2.7	1.20	6.13	.017
Age	0.86	0.32	0.97	.031
Preoperative KOOS Pain	1.1	1.04	3.45	.048

KOOS, Knee injury and Osteoarthritis Outcome Score; MCID, minimal clinically important difference; PASS, patient acceptable symptom state.

Predictors of achieving the PASS at 6 months postoperatively were male sex (OR, 2.02; P = .016), a higher preoperative KOOS Symptoms score (OR, 1.03; P = .049), and a higher preoperative KOOS Pain score (OR, 1.08; P = .040). Predictors of the PASS at 24 months were male sex (OR, 2.7; P = .017), increasing age (OR, 0.86; P = .031), and the preoperative KOOS Pain score (OR, 1.1; P = .048). There were no independent predictors of the PASS at 12 months postoperatively.

Reoperations

A total of 53 (25.7%) patients underwent a reoperation or converted to arthroplasty over the study period (Table 5). Of these patients, 6 (11.3%) converted to arthroplasty (5 to total knee arthroplasty, 1 to patellofemoral arthroplasty).

Table 5

Reoperations During the Study Period

	No. (%) of Patients
Debridement	20 (9.7)
Osteochondral allograft	9 (4.4)
Osteochondral autograft transplant	5 (2.4)
Autologous chondrocyte implant	5 (2.4)
Total knee arthroplasty	5 (2.4)
Synovectomy	3 (1.5)
Chondroplasty	2 (1.0)
De novo natural tissue graft	2 (1.0)
Loose body removal	1 (0.5)
Patellofemoral arthroplasty	1 (0.5)
Total	53 (25.7)

Discussion

The most important result of the current study was the ability to define the MCID and PASS for the IKDC and KOOS subscales at 6, 12, and 24 months postoperatively for patients undergoing primary microfracture of the knee. Furthermore, the thresholds for these psychometric measures were highest at 24 months postoperatively. These time-dependent changes suggest that the requirements for maintaining clinically significant improvements are dynamic and may become more difficult to achieve over time depending on the psychometric measure of interest and characteristics of the patient.

The current study determined that the thresholds for the MCID ranged from 8.7 to 15.2 at 6 months, 8.3 to 14.9 at 12 months, and 10.2 to 16.7 at 24 months postoperatively. For the PASS, thresholds were calculated to range from 37.5 to 88.2 at 6 months, 42.5 to 89.0 at 12 months, and 53.1 to 94.1 at 24 months. Previous studies have sought to define thresholds for clinically significant outcome improvement after various cartilage preservation procedures for the knee^16,17,24; however, none of those studies have defined specific MCID and PASS thresholds for microfracture. It is imperative to consider that thresholds for clinically significant improvements vary by both population and disease, suggesting the importance of defining microfracture-specific psychometric measures. The threshold values defined by this study are similar to those of other studies that have defined clinically significant outcome thresholds for other cartilage preservation procedures,^6,24 thus reinforcing the appropriateness of the current analysis. Future research will need to validate the use of these psychometric thresholds and further define them at longer follow-up time points.

The current study also suggests that clinically significant outcome improvement for patients who undergo the microfracture procedure for chondral defects of the knee is a dynamic and time-dependent phenomenon. Jones et al⁶ performed a systematic review and meta-analysis in order to determine which cartilage preservation procedures met the MCID for the IKDC, visual analog scale (VAS) for pain, and Lysholm scores at short-term (1-4 years), midterm (5-9 years), and long-term (≥10 years) follow-up. For the IKDC, those investigators used an established MCID of 16.7 for all cartilage preservation procedures included in the review at all follow-up time points. With respect to microfracture, they noted that patients who underwent this procedure met MCID values for all outcome scores at short- and midterm follow-up with the exception of VAS pain at midterm follow-up; however, long-term data were not available for microfracture. This demonstrates that some outcome measures may be more challenging to achieve at different postoperative time intervals. Interestingly, the MCID threshold for the IKDC (16.7) in their systematic review⁶ coincides with the microfracture-specific value defined for the IKDC in the current study at the same time point; however, agreement of MCID thresholds is not observed at earlier follow-up points. It is plausible that changes in the threshold may also exist at longer term follow-up given the high rates of failure with this procedure.²² Therefore, it is likely that the MCID has been used inappropriately in this context, because studies concerning other areas of preservation surgery of the knee, such as osteochondral autograft, osteochondral allograft, and autologous chondrocyte implant, have demonstrated that clinically significant outcome thresholds are dynamic entities that change over time.⁶

Interestingly, the proportion of patients undergoing microfracture who achieved the PASS for the outcome measures included in this study decreased between 12 and 24 months postoperatively (6 months, 67.7%; 12 months, 79.2%; 24 months, 76.1%), while the proportion of patients who achieved the MCID increased (6 months, 78.4%; 12 months, 83.9%; 24 months, 88.6%). Previous studies defining psychometric outcomes have identified the MCID as a minimum target and lower bound of outcome improvement, while the PASS has been proposed to be an intermediate outcome that is more acceptable to a patient but more challenging to achieve.¹³ This finding suggests that a larger proportion of patients begin to experience a marginal clinically important improvement at 2 years, but a smaller proportion consider this an acceptable improvement. Given that microfracture in the short- and midterm follow-up may result in early failure in select patients,^4,9,18,21,23 it is plausible that this represents an inflection point at which patients begin to experience a lower proportion of more substantial outcome improvement and progress toward worse outcomes. Future studies are warranted to determine the pattern of clinically significant outcome improvement in the mid- and long-term and to determine the rates of failures in these contexts.

Multiple pre- and intraoperative patient variables were identified that held predictive value for determining which patients undergoing microfracture surgery are likely to achieve clinically significant outcome improvement throughout various time points in the short term. Specifically, predictors of achieving the MCID included lesion size and age at surgery, whereas predictors of achieving the PASS included age, male sex, and the preoperative KOOS Symptoms and Pain scores. Few studies have sought to determine predictors of outcome after microfracture surgery. Weber et al²⁵ performed a case-control study of 101 patients who underwent primary microfracture of the knee for a focal chondral defect. The authors found that a larger lesion size was a predictor of the need for additional knee surgery at a mean of 5.66 years after microfracture surgery. They also found that increased age, BMI, and lesion size had negative correlations with lower outcome scores. These findings are in accordance with the current study, which identified lesion size and age as independent predictors of achieving clinically significant outcome improvement. Interestingly, the current study found no predictors of the MCID or PASS at 12 months postoperatively. It is possible that the high proportion of patients achieving the MCID and PASS at these time points did not allow for any specific patient factors to be identified as independent predictors of outcome, whereas the lower proportion at other time points allowed for factors to be statistically significant in the logistic regression models and subsequently identified.

The current study has limitations that should be considered in the interpretation of the results. Despite entailing a large series of patients with complete follow-up, these results represent the outcomes of patients from a single, high-volume institution, and future studies should validate the external validity of these findings. However, clinically significant outcome after microfracture surgery is poorly understood, and the results of the current study provide prognostic value for treating surgeons by providing tools to better assess how patients are progressing and by providing predictive factors to help counsel patients on expectations. Future studies will need to psychometrically analyze these outcomes as they are also commonly used tools to assess outcomes after microfracture. Another limitation is that the effect of radiographic parameters on achieving clinically significant outcome improvement, such as the Kellgren-Lawrence classification and knee alignment, was not explored. Future studies are warranted to determine whether radiographic parameters have an association with achieving the MCID and PASS after microfracture of the knee. Likewise, the effect of concomitant procedures during microfracture was not explored, as these data were missing in more than 30% of the study population, and analysis of variables with this proportion of missing data can result in misleading conclusions.¹⁹ This potential influence should also be explored in future investigations.

Conclusion

The MCID and PASS thresholds for the IKDC and KOOS in patients undergoing microfracture of the knee were dynamic, with an increasing number of patients achieving the MCID and a decreasing number of patients achieving the PASS over time. Independent predictors of achieving the MCID were lesion size and age at surgery, whereas predictors of achieving the PASS included age, male sex, and greater preoperative KOOS Symptoms and Pain scores.

Footnotes

One or more of the authors has declared the following potential conflict of interest or source of funding: B.J.C. has received research support from Aesculap/B.Braun, Arthrex, Regentis, and National Institutes of Health (NIAMS & NICHD); consulting fees from Arthrex, Acumed, Flexion Therapeutics, Vericel Corp, Geistlich Pharma North America, Smith & Nephew, Zimmer Biomet, Bioventus, Anika Therapeutics, Genzyme Corp, Regentis, and Pacira Pharmaceuticals; other financial or material support from Athletico, JRF Ortho, Carticept Medical, Lifenet Health, and Smith & Nephew; IP royalties from Arthrex and DJO; education support from Arthrex, Pacira Pharmaceuticals; and hospitality payments from GE Healthcare, Arthrex, Vericel Corp, Pacira Pharmaceuticals, Aesculap Biologics, and Depuy Synthes; and holds stock or stock options in Ossio and Regentis. A.B.Y. has received research support from Arthrex and Organogenesis, consulting fees from JRF Ortho and Aastrom Sciences Inc, and education and hospitality payments from Arthrex and Smith & Nephew. J.C. has received education payments from Arthrex and Smith & Nephew. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Submitted July 30, 2019; accepted December 4, 2019.

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