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The Lawrence D. Dorr Surgical Techniques & Technologies Award: Patient Acceptable Symptom State (PASS) in Medial and Lateral Unicompartmental Knee Arthroplasty: Does the Status of the ACL Impact Outcomes?

Published:February 02, 2022DOI:https://doi.org/10.1016/j.arth.2022.01.081

      Abstract

      Background

      The aim of this study is to determine if there is a difference in the percentage of patients who reach Patient Acceptable Symptom State (PASS) when comparing anterior cruciate ligament (ACL)-deficient and ACL-intact knees following fixed-bearing medial and lateral unicompartmental knee arthroplasty (UKA).

      Methods

      A consecutive series of 215 knees that underwent UKA (medial = 158, lateral = 57) were included in the study. The Knee Osteoarthritis Outcome Score functional score [KOOS activities of daily living (ADL)] and KOOS Sport were used as the primary outcome variables. A KOOS ADL PASS of 87.5 and KOOS Sport PASS of 43.8 were previously described for total knee arthroplasty (TKA). Failure was defined as conversion to TKA.

      Results

      There were 157 in the ACL-intact group and 58 in the ACL-deficient group. Conversion to TKA was 3.7%. The failure rate in the ACL-deficient group was 5% (3/58) and 3% (5/157) in the ACL-intact group (P = .447). The mean survival for the entire group was 18.1 years (95% confidence interval 17.6-18.6). At 10 years, the survival was 94.3% (standard error = 0.028) in the ACL-deficient group and 97.6% (standard error = 0.014) in the ACL-intact group. At a mean 10 ± 3.5 years, with 93% follow-up, 83% in the ACL-deficient group and 80% in the ACL-intact group reached PASS for KOOS ADL (P = .218). For KOOS Sport, 85% of the ACL-deficient group compared to 81% in the ACL-intact group (P = .374) reached PASS.

      Conclusion

      The ACL-deficient cohort results were not significantly different compared to ACL-intact knees in both medial and lateral compartment UKA. Fixed-bearing medial and lateral UKA resulted in low failure rate and excellent long-term outcomes.

      Keywords

      Assessing health-related quality of life (QOL) and patient satisfaction is critical in today’s medical environment and this is often accomplished with patient-reported outcome measures [
      • Ramkumar P.N.
      • Harris J.D.
      • Noble P.C.
      Patient-reported outcome measures after total knee arthroplasty. A systematic review.
      ]. Patient Acceptable Symptom State (PASS) was developed to evaluate an individual patient’s clinical status at a given time point. PASS has been previously introduced in the literature to evaluate patient satisfaction with arthritis, cartilage repair surgery, and knee arthroplasty, as an easy way to understand patient satisfaction with symptoms after a time period [
      • Chahal J.
      • Lansdown D.A.
      • Davey A.
      • Davis A.M.
      • Cole B.J.
      The clinically important difference and patient Acceptable symptomatic state for commonly used patient-reported outcomes after knee cartilage repair.
      ,
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ,
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ,
      • Maksymowych W.P.
      • Richardson R.
      • Mallon C.
      • van der Heijde D.
      • Boonen A.
      Evaluation and validation of the patient acceptable symptom state (PASS) in patients with ankylosing spondylitis.
      ,
      • Plancher K.D.
      • Shanmugam J.P.
      • Brite J.E.
      • Briggs K.K.
      • Petterson S.C.
      Relevance of the tibial slope on functional outcomes in ACL-deficient and ACL intact fixed-bearing medial unicompartmental knee arthroplasty.
      ,
      • Plancher K.D.
      • Brite J.E.
      • Briggs K.K.
      • Petterson S.C.
      Patient-acceptable symptom state for reporting outcomes following unicompartmental knee arthroplasty: a matched pair analysis comparing UKA in ACL-deficient versus ACL-intact knees.
      ,
      • Tubach F.
      • Ravaud P.
      • Baron G.
      • Falissard B.
      • Logeart I.
      • Bellamy N.
      • et al.
      Evaluation of clinically relevant states in patient reported outcomes in knee and hip osteoarthritis: the patient acceptable symptom state.
      ].
      Historically, anterior cruciate ligament (ACL) deficiency, with radiographic osteoarthritis (OA) at 5-year to 15-year follow-up, has been a contraindication to unicompartmental knee arthroplasty (UKA) as a result of higher failure rates [
      • Kiapour A.M.
      • Murray M.M.
      Basic science of anterior cruciate ligament injury and repair.
      ,
      • Cinque M.E.
      • Dornan G.J.
      • Chahla J.
      • Moatshe G.
      • LaPrade R.F.
      High rates of osteoarthritis develop after anterior cruciate ligament surgery: an analysis of 4108 patients.
      ,
      • Goodfellow J.W.
      • Kershaw C.J.
      • Benson M.K.
      • O’Connor J.J.
      The Oxford knee for unicompartmental osteoarthritis. The first 103 cases.
      ,
      • Kozinn S.C.
      • Scott R.
      Unicondylar knee arthroplasty.
      ]. Recent reports with fixed-bearing implants have challenged this contraindication and described success in the ACL-deficient knee with medial compartment OA [
      • Plancher K.D.
      • Brite J.E.
      • Briggs K.K.
      • Petterson S.C.
      Patient-acceptable symptom state for reporting outcomes following unicompartmental knee arthroplasty: a matched pair analysis comparing UKA in ACL-deficient versus ACL-intact knees.
      ,
      • Boissonneault A.
      • Pandit H.
      • Pegg E.
      • Jenkins C.
      • Gill H.S.
      • Dodd C.A.
      • et al.
      No difference in survivorship after unicompartmental knee arthroplasty with or without an intact anterior cruciate ligament.
      ,
      • Kikuchi K.
      • Hiranaka T.
      • Kamenaga K.
      • Hida Y.
      • Fujishiro T.
      • Okamoto K.
      Anterior cruciate ligament deficiency is not always a contraindication for medial unicompartmental knee arthroplasty: a retrospective study in a nondesigner’s Japanese hospital.
      ,
      • Mancuso F.
      • Dodd C.A.
      • Murray D.W.
      • Pandit H.
      Medial unicompartmental knee arthroplasty in the ACL-deficient knee.
      ]. Limited literature is available in the ACL-deficient knee following lateral UKA. To our knowledge, this is the first report to use PASS as a criterion to evaluate outcomes following nonrobotic, fixed-bearing lateral UKA.
      With an increased focus on patient satisfaction as a measure of success following surgical treatment, PASS provides an excellent way to determine the success of UKA. The primary aim of this study is to evaluate outcomes following nonrobotically-assisted, fixed-bearing medial and lateral UKA and to determine if there is a difference in the percentage of patients who reach PASS when comparing ACL-deficient to ACL-intact knees. Each subgroup, lateral and expanded medial cohorts, was additionally analyzed for outcome scores (PASS) and compared to a reference PASS for total knee arthroplasty (TKA).

      Methods

      Following Institutional Review Board approval (Quorum Protocol #33949), patients were identified who underwent UKA by the senior author between 2000 and 2016. A consecutive series of 215 knees that underwent medial UKA (n = 158) or lateral UKA (n = 57) were included in the study. Data on a small ACL-deficient (38 patients) matched cohort that underwent medial UKA have been previously published [
      • Plancher K.D.
      • Brite J.E.
      • Briggs K.K.
      • Petterson S.C.
      Patient-acceptable symptom state for reporting outcomes following unicompartmental knee arthroplasty: a matched pair analysis comparing UKA in ACL-deficient versus ACL-intact knees.
      ]. Patients were excluded if they had an ACL reconstruction within the 5 years prior to UKA, incomplete data, or refused to participate. No patients were excluded due to comorbidities, including diabetes, hypertension, and heart disease. Patella Outerbridge OA grade I to IV in any facet was not an exclusion criterion, nor was Outerbridge grade I or II OA in the opposite compartment (Fig. 1).
      Figure thumbnail gr1
      Fig. 1Diagram showing patients included in study. UKA, unicompartmental knee arthroplasty.
      Patients with an absent, torn, or nonfunctioning (eg, no restraint on probing) ACL at arthroscopy were recorded and defined as ACL-deficient. The Outerbridge grade of the patella, trochlear groove, and opposite compartment were all recorded. Patients with an ACL-deficient knee were compared with patients who underwent UKA with an ACL-intact knee. Failure was defined as conversion to TKA.

      Surgical Technique

      All patients underwent fixed-bearing UKA (Zimmer Unicompartmental High Flex Knee System; Smith & Nephew, Memphis, TN). UKA was performed using an intramedullary technique as previously reported with a patelloplasty using a medial parapatellar incision for the medial UKA [
      • Dunn A.S.
      • Petterson S.C.
      • Plancher K.D.
      Unicondylar knee arthroplasty: intramedullary technique.
      ]. A lateral approach with a lateral parapatellar incision was used to minimize incision length, soft tissue dissection, and allow for greater visualization and more accurate placement of the prothesis in the lateral UKA cohort. In ACL-deficient knees, the tibia was cut at 0° of posterior tibial slope to minimize anterior tibial translation.

      Clinical and Radiological Assessment

      Preoperative and postoperative physical examination included knee range of motion measurements, ligamentous examination, and assessment of varus or valgus deformity. Standard radiographic evaluation was performed preoperatively and postoperatively that included anteroposterior, Rosenberg [
      • Rosenberg T.D.
      • Paulos L.E.
      • Parker R.D.
      • Coward D.B.
      • Scott S.M.
      The forty-five-degree posteroanterior flexion weight-bearing radiograph of the knee, 1988 posteroanterior flexion weight-bearing radiograph of the knee.
      ], Merchant [
      • Kujala U.M.
      • Jaakkola L.H.
      • Koskinen S.K.
      • Taimela S.
      • Hurme M.
      • Nelimarkka O.
      Scoring of patellofemoral disorders.
      ], and Hughston views [
      • Carson W.G.
      • James S.L.
      • Larson R.L.
      • Singer K.M.
      • Winternitz W.W.
      Patellofemoral disorders: physical and radiographic evaluation. Part II: radiographic examination.
      ], as well as a 3-foot (full limb) long-standing radiograph [
      • Graden N.R.
      • Dean R.S.
      • Kahat D.H.
      • DePhillipo N.N.
      • LaPrade R.F.
      True mechanical alignment is found only on full-limb and not on standard anteroposterior radiographs.
      ,
      • Plancher K.P.
      • Brite J.
      • Briggs K.K.
      • Petterson S.C.
      Patients with kinematically-aligned fixed-bearing medial unicompartmental knee arthroplasty have superior outcomes and return to activity at mean ten-year follow-up.
      ] used to assess limb alignment and the status of the joint line and prosthesis.

      Clinical Assessment With Patient Acceptable Symptom State/Outcome Measures

      The Knee Osteoarthritis Outcome Score (KOOS) function of daily living (KOOS ADL) and KOOS Sport were used as the primary patient-reported outcome variables. A KOOS ADL PASS of 87.5 and KOOS Sport PASS of 43.8 have been previously defined in patients with TKA [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ]. Other outcome measures included KOOS Pain, KOOS QOL, and Lysholm. The PASS threshold for KOOS Pain was 87 and 66 for KOOS QOL, as defined in TKA [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ]. The KOOS Symptom subscale was not used due to lack of reliability in UKA patients [
      • Plancher K.P.
      • Matheny L.M.
      • Briggs K.K.
      • Petterson S.C.
      Reliability and validity of the knee injury and osteoarthritis outcome score (KOOS) in patients undergoing unicompartmental knee arthroplasty.
      ]. The PASS threshold value for the Lysholm score is reported as 70 [
      • Chahal J.
      • Lansdown D.A.
      • Davey A.
      • Davis A.M.
      • Cole B.J.
      The clinically important difference and patient Acceptable symptomatic state for commonly used patient-reported outcomes after knee cartilage repair.
      ].

      Statistical Analysis

      Statistical analysis was performed using SPSS (version 27.0, IBM Inc, Chicago, IL). Descriptive statistics (arithmetic mean, standard deviation, range) were calculated using standard formulae. For comparison between categorical data, the Fisher’s exact test was used. To determine if data departed from normal distribution, the Kolmogorov-Smirnov test was used. For data that were not normally distributed, nonparametric univariate analysis was performed using the Mann-Whitney U-test for 2-group comparisons and Wilcoxon signed-rank test for paired samples. For normally distributed data, the independent t-test was used for 2-group comparisons. Survivorship was evaluated using a Kaplan-Meier curve.

      Results

      A consecutive series of 215 knees underwent UKA and were included in the study. There were 158 medial UKAs and 57 lateral UKAs. There were 157 in the ACL-intact group and 58 in the ACL-deficient group. Demographics are shown in Table 1.
      Table 1Demographics of Patients With UKA Grouped by ACL-Deficient and ACL-Intact Knees.
      DemographicN = 215
      ACL-Deficient (N = 58)ACL-Intact (N = 157)P-Value
      Age (y)65 ± 1065 ± 11.966
      Gender (female:male)33:2586:71.253
      BMI (kg/m2)28 ± 428 ± 5.357
      ACL, anterior cruciate ligament; BMI, body mass index; UKA, unicompartmental knee arthroplasty.
      No superficial or deep infections were reported in any patient as defined by Center of Disease Control guidelines [
      • Matzon J.L.
      • Lebowitz C.
      • Graham J.G.
      • Lucenti L.
      • Lutsky K.
      • Beredjiklian P.K.
      Risk of infection in trigger finger release surgery following corticosteroid injection.
      ]. The failure rate in the ACL-deficient group was 5% (3/58), and 3% (5/152) in the ACL-intact group (P = .447). Ten-year survivorship is shown in Table 2. Survival curve for all patients is shown in Figure 2. Conversion to TKA occurred in 8 patients (Table 3).
      Table 2Survivorship at 10 y Compared Between ACL-Deficient and ACL-Intact Knees in Medial and Lateral UKA.
      Patient Group10-y Survivorship [95% Confidence Interval]Mean Survivorship (y) [95% Confidence Interval]P-Value
      All Patients97% [SE = 0.014]18.2 [17.8-18.7]
       ACL-deficient94.3% [SE = 0.028]18 [17.1-19].485
       ACL-intact97.6% [SE = 0.014]17 [16.6-17.5]
      Lateral UKA
      Survival time between medial and lateral was not significantly different (P = .506).
      96.1% [SE = 0.027]16 [15.3-17.1]
       ACL-deficient83% [SE = 0.113]14 [11.5-16.4].087
       ACL-intact100%16 [15.7-17.4]
      Medial UKA
      Survival time between medial and lateral was not significantly different (P = .506).
      97% [SE = 0.016]
       ACL-deficient97.6% [SE = 0.024]18 [17.6-19.2].699
       ACL-intact96.7% [SE = 0.019]17 [16.5-17.6]
      ACL, anterior cruciate ligament; UKA, unicompartmental knee arthroplasty; SE, standard error.
      a Survival time between medial and lateral was not significantly different (P = .506).
      Figure thumbnail gr2
      Fig. 2Kaplan-Meir survival curve. Blue line represents ACL-deficient group compared to ACL-intact group (green line). ACL, anterior cruciate ligament.
      Table 3Description of Revised UKA to TKA (n = 8) by ACL Status.
      ACLPatientYears to TKAACLReason for FailureAge at Surgery (y)GenderBMI
      IntactMedial2IntactTechnical error60Female40.7
      IntactMedial2IntactTechnical error68Female28.7
      IntactMedial2.5IntactTechnical error63Female23.4
      IntactMedial9IntactTrauma59Female40.0
      IntactLateral12IntactTrauma62Male28.7
      DeficientMedial10.5DeficientTrauma74Male32
      DeficientLateral1DeficientTechnical error72Female28.4
      DeficientLateral6.8DeficientTrauma69Female27
      TKA, total knee arthroplasty; ACL, anterior cruciate ligament; BMI, body mass index.
      Of the remaining 207 of the 215 patients in the series, follow-up was obtained on 192 (93%) patients at an average of 10 ± 3.5 years (range 4-19). There were no significant differences on any outcome scores between ACL-intact and ACL-deficient at follow-up (Table 4).
      Table 4Postoperative Patient-Reported Outcome Scores in ACL-Deficient and ACL-Intact UKA With Percentage of Patients Reaching PASS.
      Outcomes ScoreACL-Deficient UKA (N = 52)ACL-Intact UKA (N = 140)P-Value
      P-value for comparisons of proportions of patients meeting PASS between ACL-deficient and ACL-intact groups.
      Reference PASS TKA [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ,
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Erratum to: which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ]
      Postoperative Score%PASS
      PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Sport = 43.8; KOOS Quality of Life = 66 [3]; Lysholm = 70.3 [2].
      Postoperative Score%PASS
      PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Sport = 43.8; KOOS Quality of Life = 66 [3]; Lysholm = 70.3 [2].
      KOOS ADL93 ± 1988%90 ± 1484%.21869%
      KOOS Sport72 ± 2985%68 ± 2881%.374
      KOOS Pain91 ± 1183%89 ± 1675%.38362%
      KOOS Quality of Life80 ± 1783%82 ± 2087%.359
      Lysholm87 ± 1785%86 ± 1880%.672
      ACL, anterior cruciate ligament; UKA, unicompartmental knee arthroplasty; TKA, total knee arthroplasty; PASS, Patient Acceptable Symptom State; KOOS, Knee Osteoarthritis Outcome Score.
      a P-value for comparisons of proportions of patients meeting PASS between ACL-deficient and ACL-intact groups.
      b PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Sport = 43.8; KOOS Quality of Life = 66 [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ]; Lysholm = 70.3 [
      • Chahal J.
      • Lansdown D.A.
      • Davey A.
      • Davis A.M.
      • Cole B.J.
      The clinically important difference and patient Acceptable symptomatic state for commonly used patient-reported outcomes after knee cartilage repair.
      ].
      In the lateral UKA patients cohort, there was no significant difference in the percentage of patients who reached PASS for all KOOS subscales between the ACL-deficient group and the ACL-intact group (Table 5). There were no differences between ACL-deficient and ACL-intact groups in the medial UKA group (Table 6). There was no difference in knee flexion or extension range of motion between groups, preoperatively or postoperatively. Knee extension in the ACL-deficient group was 1° ± 2° and 0° ± 2° in the ACL-intact group, while flexion was 129° ± 10° and 130° ± 9° respectively.
      Table 5Postoperative Patient-Reported Outcome Scores in Patients With Lateral UKA in the ACL-Deficient and ACL-Intact Groups With Percentage of Patients Reaching PASS.
      Outcomes ScoreACL-Deficient UKA (n = 13)

      %PASS
      PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Quality of Life = 66 [3]; KOOS Sport = 43.8.
      ACL-Intact UKA (n = 36)

      %PASS
      PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Quality of Life = 66 [3]; KOOS Sport = 43.8.
      P-Value
      P-value for comparisons of proportions of patients meeting PASS between ACL-deficient and ACL-intact groups.
      Reference PASS TKA [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ,
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Erratum to: which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ]
      KOOS ADL85%83%.71269%
      KOOS Sport85%86%1.0
      KOOS Pain85%83%.83262%
      KOOS Quality of Life77%94%.091
      ACL, anterior cruciate ligament; UKA, unicompartmental knee arthroplasty; TKA, total knee arthroplasty; PASS, Patient Acceptable Symptom State; KOOS, Knee Osteoarthritis Outcome Score.
      a PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Quality of Life = 66 [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ]; KOOS Sport = 43.8.
      b P-value for comparisons of proportions of patients meeting PASS between ACL-deficient and ACL-intact groups.
      Table 6Postoperative Patient-Reported Outcome Scores in Patients With Medial UKA in the ACL-Deficient and ACL-Intact Groups With Percentage of Patients Reaching PASS.
      Outcomes ScoreACL-Deficient UKA (n = 39)

      %PASS
      PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Quality of Life = 66 [3]; KOOS Sport = 43.8.
      ACL-Intact UKA (n = 104)

      %PASS
      PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Quality of Life = 66 [3]; KOOS Sport = 43.8.
      P-Value
      P-value for comparisons of proportions of patients meeting PASS between ACL-deficient and ACL-intact groups. Some data were previously reported [7]; however, most has been updated with more recent follow-up.
      Reference for TKA [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ,
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Erratum to: which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ]
      KOOS ADL90%85%.48369%
      KOOS Sport85%80%.631
      KOOS Pain82%72%.51662%
      KOOS Quality of Life85%85%1.0
      ACL, anterior cruciate ligament; UKA, unicompartmental knee arthroplasty; TKA, total knee arthroplasty; PASS, Patient Acceptable Symptom State; KOOS, Knee Osteoarthritis Outcome Score.
      a PASS was defined as KOOS Pain = 87; KOOS ADL = 87.5; KOOS Quality of Life = 66 [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ]; KOOS Sport = 43.8.
      b P-value for comparisons of proportions of patients meeting PASS between ACL-deficient and ACL-intact groups. Some data were previously reported [
      • Plancher K.D.
      • Brite J.E.
      • Briggs K.K.
      • Petterson S.C.
      Patient-acceptable symptom state for reporting outcomes following unicompartmental knee arthroplasty: a matched pair analysis comparing UKA in ACL-deficient versus ACL-intact knees.
      ]; however, most has been updated with more recent follow-up.

      Discussion

      UKA is an excellent treatment option for patients with single compartment OA. In this study, PASS was utilized for analysis of outcomes for a consecutive series of nonrobotically assisted, fixed-bearing UKA implanted knees. Over 80% of patients after medial and lateral UKA achieved PASS with no differences between ACL-deficient knees and ACL-intact knees at mid-term to long-term follow-up. There were no differences in mean survival between ACL-deficient (18 years; 95% confidence interval 17.1-19 years) and ACL-intact knees (17 years; 95% confidence interval 16.6-17.5 years), which was also true for the medial and lateral UKA. Ten-year survivorship was over 94% for all groups except the lateral ACL-deficient group. We recommend careful patient selection to return patients to moderate or vigorous postoperative activities following medial or lateral UKA regardless of ACL status.
      PASS thresholds can define success following UKA, allowing patients to describe their own success and outcomes which can provide essential feedback to surgeons. The PASS thresholds for KOOS used in this study were determined on 3-year outcomes reported in the literature [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Matuszak S.J.
      • Ingelsrud L.H.
      • Nielsen C.S.
      • et al.
      Patient Acceptable symptom state at 1 and 3 Years after total knee arthroplasty: thresholds for the knee injury and osteoarthritis outcome score (KOOS).
      ]. Our results demonstrated a high percentage of patients maintaining PASS following UKA at an average 10 years postoperatively with maximum follow-up through 19 years. This study focused on the current state of the patient’s operative success at mid-term to long-term follow-up. The data show that most patients continued to have acceptable symptoms, despite increasing age and loss activity due to age. The percentage of patients reaching PASS in this study was higher (>80%) compared to patients after TKA (69%) [
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ,
      • Connelly J.W.
      • Galea V.P.
      • Rojanasopondist P.
      • Nielsen C.S.
      • Bragdon C.R.
      • Kappel A.
      • et al.
      Erratum to: which preoperative factors are associated with not attaining acceptable levels of pain and function after TKA? Findings from an international multicenter study.
      ].
      The chronic, degenerative knee with OA often has contracture, osteophytes, and noted scar tissue throughout the joint. These findings often aid to provide a stable knee in the anterior posterior plane [
      • Barge M.E.
      • Draganich L.F.
      • Pottenger L.A.
      • Curran J.J.
      Knee laxity in symptomatic osteoarthritis.
      ,
      • Dayal N.
      • Chang A.
      • Dunlop D.
      • Hayes K.
      • Chang R.
      • Cahue S.
      • et al.
      The natural history of anteroposterior laxity and its role in knee osteoarthritis progression.
      ,
      • Hernigou P.
      • Deschamps G.
      Posterior slope of the tibial implant and the outcome of unicompartmental knee arthroplasty.
      ,
      • Marshall J.L.
      • Olsson S.E.
      Instability of the knee. A long-term experimental study in dogs.
      ]. Marshall and Olsson [
      • Marshall J.L.
      • Olsson S.E.
      Instability of the knee. A long-term experimental study in dogs.
      ] found thickening of the joint capsule in the ACL-deficient dog and concluded that knees became less unstable as the thickness of the capsule increases. Barge et al [
      • Barge M.E.
      • Draganich L.F.
      • Pottenger L.A.
      • Curran J.J.
      Knee laxity in symptomatic osteoarthritis.
      ] described the loss of anteroposterior knee laxity in the osteoarthritic knee. Dayal et al [
      • Dayal N.
      • Chang A.
      • Dunlop D.
      • Hayes K.
      • Chang R.
      • Cahue S.
      • et al.
      The natural history of anteroposterior laxity and its role in knee osteoarthritis progression.
      ] showed that knees with Kellgren-Lawrence grade 4 osteoarthritis have less anteroposterior laxity compared to those with less OA. Clinically, our ACL-deficient knees often have limited signs and symptoms of anteroposterior instability. The senior author has found over 20 years that patients with single compartment knee OA often complain of either mediolateral instability or anteroposterior giving way.
      Understanding restoration of joint mechanics when performing a medial or lateral UKA is necessary to allow for a well-functioning UKA in most patients. Care should be taken with debridement and removal of osteophytes caused by adaptation of the degenerative knee. Soft tissue capsular contracture should also be approached carefully, removing only minimal tissue that might limit knee flexion and extension to avoid creating an unstable knee. Tibial bone cuts were set at 0° for all ACL-deficient knees to maintain the 0° posterior slope, whether for a medial or lateral UKA.
      Recent studies have challenged the need for an intact ACL whether undergoing a medial or lateral UKA [
      • Boissonneault A.
      • Pandit H.
      • Pegg E.
      • Jenkins C.
      • Gill H.S.
      • Dodd C.A.
      • et al.
      No difference in survivorship after unicompartmental knee arthroplasty with or without an intact anterior cruciate ligament.
      ,
      • Kikuchi K.
      • Hiranaka T.
      • Kamenaga K.
      • Hida Y.
      • Fujishiro T.
      • Okamoto K.
      Anterior cruciate ligament deficiency is not always a contraindication for medial unicompartmental knee arthroplasty: a retrospective study in a nondesigner’s Japanese hospital.
      ,
      • Engh G.A.
      • Ammeen D.J.
      Unicondylar arthroplasty in knees with deficient anterior cruciate ligaments.
      ]. Engh and Ammeen [
      • Engh G.A.
      • Ammeen D.J.
      Unicondylar arthroplasty in knees with deficient anterior cruciate ligaments.
      ] reported 94% survivorship at 6 years in ACL-deficient, fixed-bearing UKA and 93% survivorship in UKA in ACL-intact knees; however, patient-reported outcomes were not available. There is limited literature regarding the use of lateral UKA in the ACL-deficient knee. Although Engh and Ammeen [
      • Engh G.A.
      • Ammeen D.J.
      Unicondylar arthroplasty in knees with deficient anterior cruciate ligaments.
      ] included patients with lateral UKA, they identified only 6 knees and the results of the lateral UKAs were grouped with the medial results. We analyzed each group (medial vs lateral) separately in this study and did not find any differences.
      Lateral compartment OA is often seen in 2 unique patient groups: the younger, middle aged (<60 years of age) after a failed lateral meniscectomy or in patients with chronic pain after failed cartilage repair procedures and older patients (>60 years of age) with idiopathic, isolated lateral compartment OA [
      • Ollivier M.
      • Abdel M.P.
      • Parratte S.
      • Argenson J.N.
      Lateral unicondylar knee arthroplasty (UKA): contemporary indications, surgical technique, and results.
      ]. Managing patient expectations of these 2 disparate groups is challenging [
      • Lin B.J.
      • Zhang T.
      • Aneizi A.
      • Henry L.E.
      • Mixa P.
      • Walh A.J.
      • et al.
      Predictors of met expectations two years after knee surgery.
      ]. Younger patients often want to return to all sports they participated in prior to their activity-limiting knee pain, which could preoperatively span a period of >10 years of inactivity due to a failed cartilage or meniscus procedure with reluctance to undergo further surgery, or professional advice to delay an arthroplasty procedure for as long as possible [
      • Losina E.
      • Paltiel A.D.
      • Weinstein A.M.
      • Yelin E.
      • Hunter D.J.
      • Chen S.P.
      • et al.
      Lifetime medical costs of knee osteoarthritis management in the United States: impact of extending indications for total knee arthroplasty.
      ]. Older patients often desire to return to activities of daily living or recreational activities rather than vigorous contact or pivoting sports just to maintain a healthy lifestyle. Few, if any, publications exist describing results of these distinct separate populations of patients and their expectations with isolated lateral compartment OA.
      Lateral UKA in both these diverse patient groups has been shown to be a safe and effective treatment for isolated lateral OA in several studies [
      • Smith E.
      • Lee D.
      • Masonis J.
      • Melvin J.S.
      Lateral unicompartmental knee arthroplasty.
      ,
      • van der List J.P.
      • McDonald L.S.
      • Pearle A.D.
      Systematic review of medial versus lateral survivorship in unicompartmental knee arthroplasty.
      ,
      • Bonanzinga T.
      • Tanzi P.
      • Altomare D.
      • Dorotei A.
      • Iacono F.
      • Marcacci M.
      High survivorship rate and good clinical outcomes at mid-term follow-up for lateral UKA: a systematic literature review.
      ]. A comprehensive study found that the 5-year revision rate for lateral UKA was 12.9% [
      • Smith E.
      • Lee D.
      • Masonis J.
      • Melvin J.S.
      Lateral unicompartmental knee arthroplasty.
      ]. Although lateral UKA is performed less frequently than medial UKA, Smith et al [
      • Smith E.
      • Lee D.
      • Masonis J.
      • Melvin J.S.
      Lateral unicompartmental knee arthroplasty.
      ] reported no difference in the risk of revision between lateral and medial UKA at short-term and mid-term follow-ups. Van der List et al [
      • van der List J.P.
      • McDonald L.S.
      • Pearle A.D.
      Systematic review of medial versus lateral survivorship in unicompartmental knee arthroplasty.
      ] reported similar findings with a survivor rate of 93.2% over 5 years, 91.4% over 10 years, and 89.4% over 15 years in lateral UKA. A literature review also showed a mean survivorship of 88.6% and mean satisfaction of 78.5% in patients after lateral UKA with an average follow-up of 60 months [
      • Bonanzinga T.
      • Tanzi P.
      • Altomare D.
      • Dorotei A.
      • Iacono F.
      • Marcacci M.
      High survivorship rate and good clinical outcomes at mid-term follow-up for lateral UKA: a systematic literature review.
      ]. We found a 10-year survivorship of 96.1% following lateral UKA; with 83% in ACL-deficient knees and 100% in the ACL-intact group. This disparity in survivorship within this subgroup can easily be explained statistically by the limited number of patients (n = 13) in the ACL-deficient group vs the number of patients in the ACL-intact group (n = 42).
      UKA is a technically-demanding procedure with a steep learning curve, especially in the lateral UKA. We encourage the less experienced surgeon to refine their technique in the bioskills laboratory before proceeding to the operating room as well as to operate with an experienced surgeon in their first cases to avoid technical errors. Our consecutive series demonstrated the need for 4 patients to be revised to a TKA in the first 10 implanted cases. Nonetheless, this cohort demonstrated reproducible, excellent results with a very low revision rate at 10 years and a mean survivorship of 18.3 years. Success in medial UKA resulted in a 97%, 10-year survivorship for the ACL-intact and ACL-deficient knee with outstanding patient-reported outcomes. The lateral UKA demonstrated a similar survivorship and outcomes with KOOS Sport PASS in 85% in ACL-deficient and the ACL-intact group. However, the lateral ACL-deficient patients, due to the small numbers in this series, demonstrated a PASS KOOS QOL of 73% while the ACL-intact had a PASS KOOS QOL of 94%. This finding must be watched very carefully. We believe with the noted 2 different patient age groups that undergo lateral UKA, the younger patients (<60 years of age) put higher demands on their knees to return to vigorous cutting sports unlike the older (>60 years of age) group. This demand may affect long-term outcomes although our numbers are too small to prove or disprove this theory. With larger numbers in our series in the future, we hope to see this difference in PASS KOOS QOL normalize to the same rate as the ACL-intact group of patients over time. This discrepancy, although not statistically significant, highlights the need to track outcomes especially for patients undergoing lateral UKA who are ACL-deficient.
      This study has several limitations including a cohort treated by a single surgeon with the same prosthesis. This may prevent generalizability of the results to other centers or surgeons. However, the single surgeon study shows the applicability for the mid-volume surgeon, which represents the majority of orthopedic surgeons performing this procedure [
      • Liddle A.D.
      • Pandit H.
      • Judge A.
      • Murray D.W.
      Effect of surgical caseload on revision rate following total and unicompartmental knee replacement.
      ]. This was a consecutive series of patients undergoing UKA performed by the same surgeon; no other UKA implants were used by this surgeon during the study period. This study is strengthened by size of the cohort and the length of follow-up. This study reported patient-based outcomes, specifically the PASS for the KOOS ADL and Sport.
      In summary, this study utilized the PASS criterion to evaluate outcomes of nonrobotic, fixed-bearing UKA with or without an intact ACL. The ACL-deficient cohort results were not significantly different for all measures of outcomes compared to ACL-intact knees in both medial and lateral compartment UKA. Nonrobotic, fixed-bearing medial and lateral UKA in the ACL-deficient knee resulted in low failure rate (10-year survivorship over 94% in both groups) and excellent long-term outcomes, with over 80% of patients reaching PASS for KOOS ADL at a follow-up, which is superior to PASS rates in TKA of 69%.

      Appendix A. Supplementary Data

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