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Adverse Event Rates and Classifications in Medial Opening Wedge High Tibial Osteotomy Robin Martin, Trevor B. Birmingham, Kevin Willits, Robert Litchfield, Marie-Eve LeBel and J. Robert Giffin Am J Sports Med 2014 42: 1118 originally published online March 14, 2014 DOI: 10.1177/0363546514525929 The online version of this article can be found at: http://ajs.sagepub.com/content/42/5/1118

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Adverse Event Rates and Classifications in Medial Opening Wedge High Tibial Osteotomy Robin Martin,*y MD, Trevor B. Birmingham,*z§ PhD, Kevin Willits,*|| MD, FRCS(C), Robert Litchfield,*|| MD, FRCS(C), Marie-Eve LeBel,*|| MD, FRCS(C), and J. Robert Giffin,*§|| MD, FRCS(C) Investigation performed at Fowler Kennedy Sport Medicine Clinic, University of Western Ontario, London, Ontario, Canada Background: Previously reported complications in medial opening wedge (MOW) high tibial osteotomy (HTO) vary considerably in both rate and severity. Purpose: (1) To determine the rates of adverse events in MOW HTO classified into different grades of severity based on the treatments required and (2) to compare patient-reported outcomes between the different adverse event classifications. Study Design: Case series; Level of evidence, 4. Methods: All patients receiving MOW HTO at a single medical center from 2005 to 2009 were included. Internal fixation was used in all cases, with either a nonlocking (Puddu) or locking (Tomofix) plate. Patients were evaluated at 2, 6, and 12 weeks; 6 and 12 months; and annually thereafter. Types of potential surgical and postoperative adverse events, categorized into 3 classes of severity based on the subsequent treatments, were defined a priori. Medical records and radiographs were then reviewed by an independent observer. The Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores were compared in subgroups of patients based on the categories of adverse events observed. Results: A total of 323 consecutive procedures (242 males) were evaluated (age, mean 6 standard deviation, 46 6 9 years; body mass index, mean 6 standard deviation, 30 6 5 kg/m2). Adverse events requiring no additional treatment (class 1) were undisplaced lateral cortical breaches (20%), displaced (.2 mm) lateral hinge fracture (6%), delayed wound healing (6%), undisplaced lateral tibial plateau fracture (3%), hematoma (3%), and increased tibial slope 10° (1%). Adverse events requiring additional or extended nonoperative management (class 2) were delayed union (12%), cellulitis (10%), limited hardware failure (1 broken screw; 4%), postoperative stiffness (1%), deep vein thrombosis (1%), and complex regional pain syndrome (CRPS) type 1 (1%). Adverse events requiring additional or revision surgery and/or long-term medical care (class 3) were aseptic nonunion (3%), deep infection (2%), CRPS type 2 (1%), and severe hardware failure with loss of correction (1%). Additional surgery rate was 3%. Class 1 and 2 adverse events did not affect patient-reported outcomes at 6, 12, or 24 months postoperatively. Patients with class 3 adverse events had significantly lower total WOMAC scores at 6 months but not at 12 or 24 months postoperatively. Conclusion: The most common adverse event in MOW HTO requiring extended nonoperative treatment (class 2) is delayed union (12%). The rate of severe adverse events requiring additional surgery and/or long-term medical care (class 3) is low (7%). Keywords: open wedge; high tibial osteotomy; adverse events; complications

High tibial osteotomy (HTO) is a well-established treatment for varus malalignment with associated medial compartment osteoarthritis, medial compartment osteochondral defect, instability, and hyperextension thrust.2,23 Despite evidence of good long-term results after HTO, the procedure has relatively low rates of use.1,2,11,23,40 This may be partly due to reported complications associated with the procedure.13,22,24,31,37

Several techniques have been proposed for HTO. The lateral closing wedge osteotomy is the traditional approach.10 This technique, while producing good results, is associated with possible severe adverse events, including peroneal nerve palsy (up to 20%), fibula nonunion (3.8%), lateral collateral ligament insufficiency with recurrence of varus deformity (33%-68%), compartment syndrome (1.9%), and loss of bone stock that can complicate future arthroplasty.30,32,34,39 Further, it is a technically demanding procedure. Multiple bone resections and a disruption of the proximal tibiofibular joint are often required to achieve the desired correction. The medial opening wedge (MOW) HTO was developed to address these technical challenges and limit

The American Journal of Sports Medicine, Vol. 42, No. 5 DOI: 10.1177/0363546514525929 Ó 2014 The Author(s)

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complications. This technique offers minimal muscular dissection, no need for fibular osteotomy or tibiofibular joint separation, no need for peroneal nerve dissection, no bone stock alteration, and an easier adjustment of the correction in both the coronal and sagittal planes. However, this procedure presents different challenges, especially related to osseous union of the opening wedge and the need to stabilize this gap during healing. We are aware of 7 studies reporting adverse events in MOW HTO, with sample sizes ranging from 20 to 59.7,13,22,24,29,31,37 When results are reported as overall complication rates, these studies suggest that some form of adverse event occurs in 37% to 55% of cases. Importantly, the severity of adverse events in MOW HTO can range from benign findings that do not affect how patients are managed or their overall outcomes to severe complications that require further medical and/or surgical treatments. We propose that when adverse events in MOW HTO are described, a distinction between these occurrences would be useful for clinicians and patients faced with treatment options and may help in future research investigating risk factors and interventions. The objectives of the present study were (1) to determine the rates of adverse events in MOW HTO classified into different grades of severity based on the treatments required and (2) to compare patient-reported outcomes between the different adverse event classifications.

MATERIALS AND METHODS Study Design In this retrospective case series, all patients who had received a MOW HTO, with or without concomitant procedures, at the Fowler Kennedy Sport Medicine Clinic, London, Ontario, Canada, between September 2005 and August 2009 were included. A total of 323 consecutive HTOs were evaluated, performed on 292 patients. No patients were excluded. Surgeries were performed by (or under the guidance of) 1 of 4 fellowship-trained orthopaedic surgeons. All surgeries and follow-ups were undertaken at this same center. Patients were seen in clinic preoperatively, then 2, 6, and 12 weeks and 6 and 12 months postoperatively, and annually thereafter. Anteroposterior and lateral knee radiographs were performed at 6 and 12 weeks, and hip-to-ankle longstanding radiographs were performed yearly. Preoperative and postoperative assessments were

recorded in the patient’s medical record by the attending surgeon and/or the orthopaedic surgery fellow or resident. Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores were also available for a subgroup of 258 patients who agreed to take part in a separate, prospective observational cohort study evaluating long-term biomechanical and clinical outcomes.4,5 Possible surgical and postoperative adverse events were listed a priori based on previously published studies.7,13,22,24,29,31,37 In 2010, a single orthopaedic surgeon not involved in the patient’s care (R.M.) directly evaluated the original radiographs (and computed tomography [CT] scans, if ordered) to identify nonunion, delayed union, lateral hinge fractures, lateral plateau fractures, increased tibial slope, intra-articular screw, and limited or full hardware failure. The same investigator directly reviewed laboratory test results to identify positive bone graft cultures and deep infections and directly reviewed administrative records to identify readmissions and revision surgeries. This investigator also extracted other adverse events from the patient’s medical records (as recorded by the original surgeon or his or her representative [fellow/resident]). Radiographs, laboratory tests, and medical records from the first 6 weeks postoperatively were reviewed to identify wound issues (delayed wound healing, cellulitis), hematoma, surgical complications (increase in tibial slope, lateral hinge and tibial plateau fractures, neurovascular injury, intraarticular screw), compartment syndrome, positive culture for cancellous bone allograft, and general complications (urinary tract infection, cardiac and pulmonary complications). Data from the first 6 months postoperatively were reviewed to identify deep vein thrombosis (DVT), stiffness, complex regional pain syndrome (types 1 and 2), and delayed union. Data from at least 12 months postoperatively were reviewed to identity nonunion, hardware failure, and infection. Classifications for adverse events were also agreed upon a priori based on the treatments they would require. Class 1 adverse events were considered those that required no additional treatment or very minor alterations in postoperative care (eg, prophylactic medication or minor alteration in weightbearing status). Class 2 adverse events required additional or extended nonoperative treatment for a limited period of time (eg, prolonged period of partial weightbearing). Class 3 adverse events required either long-term medical treatment or additional surgery. This study was approved by the University of Western Ontario Research Ethics Board for Health Sciences Research Involving Human Subjects.

§ Address correspondence to J. Robert Giffin, MD, FRCS(C), Fowler Kennedy Sport Medicine Clinic, University of Western Ontario, London, Ontario, Canada, N6A 3K7 (e-mail: [email protected]); and Trevor B. Birmingham, PhD, Fowler Kennedy Sport Medicine Clinic, University of Western Ontario, London, Ontario, Canada, N6A 3K7 (e-mail: [email protected]). *Fowler Kennedy Sport Medicine Clinic, University of Western Ontario, London, Ontario, Canada. y Department of Orthopaedic Surgery and Traumatology, University Hospital of Lausanne, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland. z Faculty of Health Sciences, University of Western Ontario, London, Ontario, Canada. || Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada. One or more of the authors has declared the following potential conflict of interest or source of funding: T.B.B. and J.R.G. have received research funding from Arthrex Inc. This work was completed at the Fowler Kennedy Sport Medicine Clinic thanks in part to research grants from the Canada Research Chairs Program (T.B.B.), the Canadian Institutes of Health Research, and Arthrex Inc.

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TABLE 1 Patient Demographic and Clinical Characteristics (N = 323 Procedures)a Characteristic Age, mean 6 SD, y Sex, ratio (No. male/No. female) Operative side, left/right, No. Body mass index, mean 6 SD, kg/m2 American Society of Anesthesiologists score,b % (n) 1 2 3 Diabetes, % (n) Active smokers, % (n) Preoperative diagnosis, % (n) Medial OA with varus Osteochondral defect with varus Medial OA, varus and ACL deficiency, or failed ACL reconstruction Varus and multiligamentous injury Varus and posterior cruciate ligament deficiency Recurvatum and varus thrust

46.3 6 9.3 2.99 (242/81) 181/142 29.9 6 5.4

32.5 49.5 18.0 4.0 20.1

(105) (160) (58) (13) (65)

62.2 (201) 5.6 (18) 23.5 (76) 2.5 (8) 5.0 (16) 1.2 (4)

a

ACL, anterior cruciate ligament; OA, osteoarthritis. American Society of Anesthesiologists physical status classification system: 1 = a normal, healthy patient; 2 = a patient with a mild systemic disease; 3 = a patient with severe systemic disease. b

posterior border height). For AMOW HTO, the plate was placed along the anterior half of the medial aspect of the tibia. The osteotomy was filled with cancellous allograft (30 mL) mixed with vancomycin (1 g). A swab was taken from the bone graft for culture. If a patient’s religious convictions precluded the use of an allograft, either no substitute or demineralized bone matrix was used. Reconstruction of the anterior cruciate ligament (ACL), or more rarely multiligament reconstruction, was performed if indicated. Because the plate was placed along the posterior half of the medial aspect of the tibia for MOW HTO, the tibial tunnel could be drilled without any screw interference if needed. A drain was inserted. Subcutaneous tissue was closed with an interrupted No. 2-0 Vicryl suture, skin with a running No. 3-0 Monocryl suture.

Postoperative Care Patients received antibiotics for 24 hours. They were admitted for a 1-night stay in hospital. Prophylaxis for DVT was not routinely prescribed unless the patient reported a history of DVT. Patients were placed in a hinged knee brace postoperatively. Crutch use was mandated for at least 6 weeks with feather-touch weightbearing on the operative limb. With radiographic and clinical evidence of surgical site healing, protected weightbearing was permitted at 6 weeks and full weightbearing at 10 to 12 weeks. Patients started physical therapy immediately postoperatively and progressed until they demonstrated normal gait patterns determined at the physical therapist’s discretion.

Surgical Procedures Preoperative templating was performed for each case using hip-to-ankle longstanding radiographs. The general aim was to bring the weightbearing axis to 62.5% of the proximal tibia width.12 Care was taken to preserve the tibial slope, except in cases of combined posterior cruciate ligament deficiency, posterolateral corner injury, or symptomatic genu recurvatum, where slope was increased intentionally (using an anteromedial open wedge [AMOW] HTO). Preoperative antibiotics were administered. A tourniquet was used at the surgeon’s discretion. A diagnostic arthroscopy was performed unless the patient had no history of mechanical symptoms and no signs of arthritic changes in the lateral and patellofemoral compartment on preoperative radiographs. If an arthroscopy had recently been performed, it was not repeated. This was followed by MOW HTO, performed with a previously described technique.3 If the tibial osteotomy would reach the tibial tubercle, it was extended upward or downward to protect the patellar tendon insertion at the tubercle. A down cut was chosen if the correction was superior to 12.5 mm, because we suggest that this preserves patellar height with larger corrections.19 Either a secondgeneration nonlocking 4-hole Puddu plate (n = 267; Arthrex, Naples, Florida, USA) or a Tomofix locking plate (n = 56; Synthes, West Chester, Pennsylvania, USA) was used at the surgeon’s discretion. For MOW HTO, we took care to place the plate along the posterior half of the medial aspect of the tibia to create a wedge that respects the tibial slope (a wedge with an anterior border height that is half of the

Statistical Analysis The number of patients experiencing each adverse event was recorded and calculated as a percentage of the total number of patients assessed for that adverse event. Improvements in total WOMAC scores at 6, 12, and 24 months postoperatively were compared between 3 subgroups of patients (no adverse event, or adverse event class 1, 2, or 3) by use of a 2-factor time-by-group analysis of variance. Although the most relevant adverse events were also recorded separately for patients who were smokers, were diabetic, received Puddu or Tomofix plates, and underwent ligament reconstruction, the differences in rates between those subgroups were not compared statistically because of the low numbers of events. Statistics were completed using the Statistical Package for Social Sciences (IBM SPSS Statistics v 21; IBM Corp, Armonk, New York, USA).

RESULTS Demographic and clinical characteristics, including indications for osteotomy, are presented in Table 1. Thirty-one patients (10.6%) received bilateral HTO during the time period studied. Three percent (9/323) were lost to followup at 6 months, and 7.1% (22/323) were lost to follow-up at 1 year. The mean 6 standard deviation follow-up time was 39.5 6 21.6 months. Mean wedge size was 11.8 6 3.5 mm. Eighty-three percent received HTO alone,

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TABLE 2 Adverse Events in Medial Opening Wedge High Tibial Osteotomy Rate of Adverse Events, %

No. of Patients/Total

1.00 2.00 2.80 2.80 5.60 5.60 19.80

3/297 6/303 9/323 9/323 18/323 18/323 64/323

0 1.30 1.30 1.30 4.00 9.60 12.00

0/322 4/314 4/314 4/314 12/301 31/322 37/307

0 0 0 0 0 0 1.00 1.30 1.70 3.20

0/322 0/322 0/322 0/323 0/322 0/323 3/301 4/314 5/301 10/317

Class 1: No additional treatment required Increased tibial slope 10° Positive culture for cancellous bone allograft Undisplaced (\2 mm) lateral tibial plateau fracture Hematoma Delayed wound healing Displaced (2 mm) lateral hinge fracture Undisplaced (\2 mm) lateral hinge fracture Class 2: Short-term nonoperative treatment required Urinary tract infection Postoperative stiffness Deep vein thrombosis Complex regional pain syndrome type 1 Limited hardware failurea Cellulitis Delayed unionb Class 3: Additional surgery or long-term nonoperative treatment required Cardiac complication Pulmonary complication Neurovascular injury Intra-articular screw Compartment syndrome Displaced lateral tibial plateau fracture Hardware failure with loss of correction Complex regional pain syndrome type 2 Deep infection Aseptic nonunion a

No loss of correction. Insufficient bone healing on 3-month postoperative radiograph to recommend discharge of crutch use but achieved within 6 months postoperatively. b

whereas 16.1% were performed with primary (10.5%; 34/323) or revision (5.6%; 18/323) ACL reconstruction. Multiligament reconstruction was performed in 1% of cases (3/323), with posterolateral corner reconstruction for 2 of them and medial collateral ligament and ACL reconstruction for 1. Diagnostic arthroscopy was undertaken before HTO in 78.6% of cases (254/323). The tourniquet was inflated in 48.6% of cases (157/323) for 61.6 6 33.8 minutes. The osteotomy was extended with a tibial tubercle osteotomy in 26.3% of cases (85/323), with 13 up cuts (to protect the patellar tendon insertion) and 72 down cuts (to maintain patellar height with larger corrections). HTOs were stabilized with a nonlocking plate in 82.7% (267/323) and a locking plate in 17.3% (56/ 323) depending primarily on preference of the surgeon and/ or cases with very large corrections (.17.5 mm). The osteotomy was filled with cancellous allograft in 93.8% (303/323) and demineralized bone matrix in 2.2% (7/323). It was left unfilled in 4.0% (13/323). Adverse event rates are summarized in Table 2 and are described in detail below.

Class 1 Adverse Events A breach in the lateral cortex was identified in 19.8% of cases (64/323) after a thorough review of patient images. Thirty-eight were identified on intraoperative fluoroscopy and 26 were identified on at least 1 of the postoperative

films. Lateral hinge fractures were considered undisplaced if translation between fragments was less than 2 mm. No changes in postoperative care were made. Undisplaced lateral tibial plateau fracture occurred in 2.8% of cases (9/323). These fractures were considered undisplaced if the intra-articular step-off was less than 2 mm. They did not require fixation. Patients were asked to remain feather-touch weightbearing for 8 weeks instead of 6 weeks, followed by 6 weeks of protected weightbearing. Displaced lateral hinge fracture (translation between fragments 2 mm) occurred in 5.6% of cases (18/323). No additional internal fixation was undertaken except in 1 case (lateral approach and staple). Patients were asked to remain feather-touch weightbearing for 8 weeks instead of 6 weeks, followed by 6 weeks of protected weightbearing. Cancellous bone graft culture was positive in 2% of cases (6/303). Four were considered contaminants in light of the identified bacteria (Bacillus cereus, Propionibacterium) or fungi (Chaetomium globosum, Alternaria) and were not treated. Two received prophylactic treatment (Staphylococcus warneri, treated with intravenous vancomycin for 10 days; Candida glabrata, treated with oral fluconazole for 8 weeks). None of these patients developed a superficial or deep infection. Delayed wound healing occurred in 5.6% of cases (18/ 323). Wound healing was considered delayed if at the

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2-week postoperative appointment, the wound presented with persistent serous drainage, skin necrosis, or dehiscence, without redness, warmth, or swelling on the edges. Superficial dehiscence rate was 2.5% (8/323). Persistent serous drainage occurred in 7 cases. Necrosis of skin on the edges from the scar was rare (0.9%; 3/323). None of these cases were associated with infection. These patients were treated with repeated local wound care and dressing. None received antibiotics or additional surgery. Hematoma occurred in 2.8% of cases (9/323). An increase in tibial slope 10° or greater occurred in 1.0% of cases (3/297). We did not include AMOW HTOs (16/323) in this calculation because the tibial slope was increased intentionally in those cases.

decreased symptoms to recommend full weightbearing were achieved after 4.3 6 0.6 months. Limited hardware failure occurred in 4.0% of cases (12/ 301), defined by failure of 1 screw, either broken (8/12) or backing out (4/12), with no loss of correction seen on sequential hip-to-ankle radiographs. Deep vein thrombosis occurred in 1.4% of cases (4/314), diagnosed based on symptoms and clinical examination and confirmed by ultrasound. Three cases occurred on the operated limb at the level of the popliteal vein, and 2 of them extended into the superficial femoral vein. History of DVT was found for 1 of these patients. One case occurred at the level of the internal jugular vein, after use of a peripherally inserted central catheter to administer antibiotics for treatment of a deep infection. None resulted in pulmonary embolism.

Class 2 Adverse Events Superficial cellulitis occurred in 9.6% of cases (31/322). It was defined by redness, swelling, and warmth on the edges of the wound, with or without serous drainage. Patients were treated with an oral antibiotic (cefazolin 500 mg every 8 hours or, if an allergy was present, clindamycin 300 mg every 6 hours) for 2 weeks. None progressed to deep infection. Thirteen percent of these (4/31) had drainage. Postoperative stiffness occurred in 1.3% of cases (4/314), defined as either a knee flexion contracture greater than 10° or an inability to flex the knee 90°. All cases required additional physiotherapy and readmission for manipulation under anesthesia. None required arthrolysis. Three of the 4 cases had combined primary ACL reconstruction with the MOW HTO. None of the patients receiving revision ACL or multiligament reconstruction with the MOW HTO experienced postoperative arthrofibrosis. Complex regional pain syndrome (CRPS) was defined according to the International Association for the Study of Pain. A CRPS type 1 (1.3%; 4/314) clinical diagnosis was completed by a bone scan for 3 of the 4 cases. Symptoms lasted between 6 and 10 weeks and resolved spontaneously. Suspected delayed union based on postoperative radiographs occurred in 12.0% of all cases (37/307). Delayed union was operationally defined as insufficient evidence of radiographic union to discontinue crutch use 3 months postoperatively but subsequently present within 6 months postoperatively. In 37.8% of these cases (14/37), delayed union was confirmed with CT scan. The rate of delayed union was 18.3% (11/60) for active smokers, 15.6% (7/45) for ex-smokers, and 9.4% (19/202) for nonsmokers. The rate was 27.3% (3/11) for diabetic patients and 11.5% (34/ 296) for nondiabetic patients. The rate was 13.7% (35/ 256) for patients with a nonlocking (Puddu) plate and 3.9% (2/51) for patients with a locking (Tomofix) plate. The rate was 11.9% (30/252) for patients receiving HTO alone, 8.8% (3/34) when HTO was performed with primary ACL reconstruction, and 19.0% (4/21) when HTO was performed with revision ACL or multiligament reconstruction. Nonoperative treatment was applied in all cases, with an extended period of protected weightbearing. The Exogen Ultrasound Bone Healing System (Smith & Nephew, London, UK) was used in 1 case. Sufficient consolidation and

Class 3 Adverse Events There were no cardiac complications (including myocardial ischemia and infarction, auricular and ventricular arrhythmias), pulmonary events (including pneumonia, pulmonary embolism), or anesthesia-related complications. No deaths occurred with a minimum of 24 months of postoperative follow-up. Deep infection occurred in 1.7% of cases (5/301), defined by the association of local signs (skin redness, warmth, swelling, with or without sinus and purulent drainage) with elevated erythrocyte sedimentation rate and C-reactive protein (20 mm/h and 10 mg/L, respectively). We isolated methicillin-sensitive Staphylococcus aureus (MSSA; 3/5), Staphylococcus lugdunensis (SL; 1/5), and Streptococcus agalactiae (SA; 1/5). Three cases were diagnosed at 3 months (MSSA n = 2 and SL n = 1), and other cases presented late at 6 months (MSSA; 1/5) and 1 year (SA; 1/5) postoperatively, possibly after a systemic illness or bacteremia. They all received irrigation and debridement (I/D), hardware removal (immediately or delayed), and intravenous antibiotics through a peripherally inserted central catheter for 6 weeks. If hardware could be removed immediately (3/5; union achieved), no recurrence was noted. A single I/D was sufficient. If hardware removal had to be delayed (2/5; union not achieved; infection at 3 months), infection recurred. The I/Ds were repeated until hardware was removed. The criteria for CRPS type 2 were present in 1.3% of cases (4/314). Two of these 4 patients had a previous knee dislocation. One had received multiligament reconstruction before HTO. One received ACL reconstruction at the time of the HTO. One had initially sustained a DVT after surgery, successfully treated with heparin and then aspirin. All 4 patients had continuing pain, allodynia, or hyperalgesia that could not be explained by the existence of any other condition at their latest follow-up appointment (12-48 months). Pain was diffuse, affecting the surgical leg and foot for 3 patients. Pain was limited to the region of the surgical scar (potential neuroma) for the remaining patient. Diagnosis of CRPS type 2 was a diagnosis of exclusion. Nerve conduction studies were performed in 2 cases. A superficial peroneal and tibial nerve axonal injury was diagnosed in 1 case. Three of the

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4 patients were treated with narcotics. One was treated with pregabalin and a tricyclic antidepressant. Aseptic nonunion occurred in 3.2% of cases (10/317), defined as the absence of radiographic union at 6 months with persistence of load-dependent pain over the osteotomy. All cases affected the tibial metaphysis. We had no case of tibial tubercle osteotomy nonunion. Diagnosis was confirmed on a CT scan in 80% (8/10). For the remaining (2/ 10), this was not needed because a progressive collapse of the HTO and hardware failure could be seen on radiographs. The rate of nonunion was 4.8% (3/62) for active smokers, 4.3% (2/47) for ex-smokers, and 2.4% (5/208) for nonsmokers. The rate was 16.6% (2/12) for diabetic patients and 2.6% (8/305) for nondiabetic patients. The rate was 3.4% (9/265) for patients with a nonlocking (Puddu) plate and 1.9% (1/52) for patients with a locking (Tomofix) plate. We observed no case of nonunion for patients receiving HTO with either revision ACL or multiligament reconstruction. For all patients, we attempted nonoperative treatment of continued protected weightbearing with crutches. The Exogen Ultrasound Bone Healing System was added for 2 patients. Seven of the 10 patients achieved bone union with this regimen, at 8.3 6 2.0 months (minimummaximum, 7-12 months). Three of the 10 required additional surgery (included below), 2 for hardware failure and loss of correction (at 7 and 14 months) and 1 for persistent symptomatic nonunion (at 6 months). Hardware failure with loss of correction occurred in 1% of patients (3/301), who admitted noncompliance with the postoperative weightbearing protocol. One patient had a breakage of the plate itself (Puddu) with collapse but ended up with union. He refused additional surgery. The 2 others had a failure of the 2 distal cortical screws of the plate (Puddu), with collapse and nonunion. They were revised. There were no cases of popliteal vascular injury, no tibial or peroneal nerve injury, no compartment syndrome, no complication related to anesthesia, no displaced lateral plateau fracture, no tibial tubercle fracture, and no intraarticular screw.

Additional Surgery and Readmissions A total of 4.0% of cases (12/300) required readmission for complications. Reasons were surgery for deep infection (5 patients), surgery for aseptic nonunion (3 patients), and manipulation under anesthesia (4 patients). The mean time to readmission was 7.1 6 3.9 months (minimummaximum, 2-14 months). Additional surgery rate was 2.7% (8/300). Surgeries were performed for deep infection in 5 cases and aseptic nonunion with or without hardware failure and loss of correction in 3 cases. Mean time to surgery was 6.6 6 4.3 months (minimum-maximum, 2-14 months). Surgery for infection was performed at a mean 5.2 6 4.1 months (minimum-maximum, 2-12 months). For 2 of these patients, multiple procedures (2 I/Ds each) were needed as the hardware had to be kept in place until union was achieved. These 2 patients had a total of 3 additional surgeries each. An additional surgery for nonunion was performed at a mean of 9 6 4.4 months (minimum-

Figure 1. Means and standard deviations for total Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores pre- and postoperatively for patient groups based on classification of worse adverse event. *Patients with class 3 adverse events had significantly lower scores than all other groups at 6 months postoperatively (P \ .05) but not at 12 and 24 months. **Preoperative scores for patients with class 2 and 3 adverse events were significantly lower than those for patients with class 1 or no adverse events (P \ .05). maximum, 6-14 months). Revisions were performed using a locking plate with bone graft (iliac crest, n = 1; cancellous allograft, n = 2). It was supplemented either with BMP-2 (Infuse, Medtronic, Minneapolis, Minnesota, USA), BMP7 (OP-1, Novos, Stryker, Natick, Massachusetts, USA), or platelet-rich plasma (Arthrex ACP system, Naples, Florida, USA).

Effect on Patient-Reported Outcome There was a significant time-by-group interaction, indicating that the improvement in WOMAC scores over time after surgery depended on the classification group for worse adverse event (P = .035). Post hoc tests indicated that patients with class 3 adverse events had significantly lower scores than all other groups at 6 months postoperatively (P \ .05) but not at 12 and 24 months (Figure 1). Also, the preoperative WOMAC scores for patients with class 2 and 3 adverse events were significantly lower than those of patients with class 1 or no adverse events (P \ .05) (Figure 1).

DISCUSSION The present study provides a comprehensive description of adverse event rates, categorized by their severity, and their effect on patient-reported outcomes. Overall, the most severe individual adverse events that require substantial changes in treatment are very low (Table 2). Furthermore, there were no systemic complications, no deaths, no cardiac or respiratory events, no stroke, and no urinary

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tract infections. Similar results were presented by Sikorski et al,27 who further reported that these systemic risks are greater with total knee arthroplasty than with HTO. In fact, the only systemic complication reported after HTO in the literature to date is pulmonary embolism, with rates between 0.2% (1/386) and 1.7% (1/58).3,13,14 The very low rates for severe adverse events in HTO are encouraging; however, the low number of events also poses challenges for investigating their effect, risk factors, and potential interventions. We propose that grouping multiple adverse events into the present classifications may help in that regard. For example, the present findings suggest that patients with class 3 adverse events experience less improvement in patient-reported outcomes at 6 months postoperatively but ‘‘catch up’’ by 12 to 24 months (Figure 1). Class 1 and 2 adverse events do not appear to affect these patient-reported outcomes by 6 months postoperatively (Figure 1). Overall, although any adverse event is important, the present findings suggest that the goal of improving knee-specific pain and function is achieved in HTO even when an adverse event exists. The notable exception in the present sample is the 4 patients who met the criteria for CRPS type 2. These patients had long-term symptoms that required long-term pain medication use. Class 1 adverse events were most frequent, with a total rate of 39.6%. We observed delayed wound healing in 5.6% of cases. This is higher than previously reported rates (3.4%, 2/58, Esenkaya et al13; 0%, 0/50, Yacobucci et al41; 0.4%, 2/386, Floerkemeier et al14). This might be explained by the fact we included cases with persistent serous drainage and mild skin necrosis along the incision (with secondary eschar), in addition to those with superficial wound dehiscence. Undisplaced lateral tibial plateau fracture occurred in 2.8% of patients. Two previous studies reported rates at 8.6% (5/58) and 19% (14/74).3,13 Other studies have not differentiated displaced or undisplaced fractures and have reported rates between 2.2% and 9%.9,13,24,26,30 We did not observe any case of tibial plateau fracture occurring in the postoperative period, but Schroter et al26 did (3%, 1/35). We observed hematoma in 2.8% of cases, which is consistent with previous reports,6,18,26,31,36 where rates ranged from 1.5% (4/262) to 3.9% (2/51).6,14,15,22,27,32 The postoperative anticoagulation regimen and the use of a drain are not consistently reported. Also, some of these studies only considered hematomas that required additional surgery.18,31 None of the present cases with hematoma needed surgery. Lobenhoffer et al18 and Spahn31 reported additional or revision surgery rates of 1.5% (4/262) and 3.6% (2/55), respectively, for irrigation and debridement of hematoma. Cellulitis was present in 9.6% of cases, when defined as the presence of redness, warmth, and swelling around the scar only, with no purulent drainage or significant elevation of the erythrocyte sedimentation rate and C-reactive protein. These patients were treated with a short course of antibiotics as a precaution. It should be noted that these data for infections were determined from the review of clinic notes dictated by medical professionals with varying levels of training. Valkering et al36 reported 10% (4/40) and Takeuchi et al33 reported 1% (1/104) with superficial infection. It is difficult to compare results, as definitions for cellulitis vary.

In the present study, 19.8% of patients had an undisplaced breach of the lateral tibial cortex and 5.6% had a displaced fracture. There are no previous studies that differentiate displaced and undisplaced lateral hinge fracture rates. Previous reports vary considerably, with rates ranging from 6% (3/50) to 34.8% (15/43).7,13,17,21,22,24,31,33,41 Diagnosis is influenced by the quality and direction of radiographs. In the present study, only 59.4% could be identified at time of surgery. Similarly, Miller et al22 diagnosed 50% of their cases on postoperative radiographs. Undisplaced lateral hinge fractures received no change in postoperative management, whereas patients with displaced fractures had their period of feather-touch weightbearing extended from 6 to 8 weeks. The effect of lateral hinge fracture is unclear. Yacobucci et al41 and Meidinger et al21 reported that lateral hinge fracture was a risk factor for nonunion. Other factors such as diabetes, wedge size, and plate type are also likely involved, and a multivariate analysis is required to confidently assess the effect of these fractures on delayed union and nonunion. Class 2 adverse events were less common (29.5%). Rate of DVT was 1.3% (4/314). We did not prescribe DVT prophylaxis. A tourniquet was only used in 1 of these 4 cases. Previously reported rates vary from 1.8% (1/55) to 4.3% (2/46), although the use of prophylaxis and/or tourniquet varies.6,22,31 Esenkaya and Elmali13 reported a rate of 3.4% (2/58) for MOW HTO with a 10- to 12-day course of low-molecular-weight heparin, while van den Bekerom et al37 and Miller et al22 both reported 0% (0/20 and 0/40, respectively) with the use of 6 weeks of prophylaxis. For comparison, Tunggal et al34 suggested in a review of medial closing wedge HTO that DVT occurs in 2% to 5% of cases and that use of a tourniquet did not have a significant effect. Our study may underestimate the true DVT rate as we only considered symptomatic DVTs. Turner et al35 estimated the DVT rate at 41% (34/84) after medial closing wedge HTO based on venography at 7 to 10 days postoperatively. Delayed union (ie, insufficient healing on the 3-month postoperative radiograph, subsequently evident by the 6month radiograph) was present in 12.0% of cases. None required additional surgery. Yacobucci et al41 reported that 18% of cases (9/50) had delayed union, Floerkemeier et al14 reported a rate of 1.5%, and Lobenhoffer et al18 reported a rate of 0.8% (2/262). This question is of particular interest, as one of the theoretical drawbacks of the MOW technique when compared with the lateral closing wedge (LCW) technique is a risk of delay in union due to the gap created at the time of the surgery. In a recent literature review, Tunggal et al34 reported delayed union in 4% to 8.5% of cases with LCW HTO. The rate of aseptic nonunion was much lower (3.2%), but this is still of concern given the requirement of additional surgery. A recent review of the literature estimates the risk of nonunion with LCW HTO at 1% to 5%.34 Importantly, definitions of delayed union vary considerably among authors. Some authors14,18 used only reported patients undergoing revision to circumvent nonunion at a later stage, while other authors have reported all cases.41 Further, it is likely that complete osseous consolidation after HTO may take longer than 6 months, especially in cases when rigid fixation systems such as the Tomofix plate are

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used. This is why the present definition of delayed union was based on the clinician’s perception of the extent of bone healing with respect to the recommendation of discontinuing crutch use. This definition would tend to overestimate rates of delayed union in the present study. The time to complete consolidation after MOW HTO is presently unclear. Class 3 adverse events were infrequent (7.2%). We did not report any vascular injury, whereas Chae et al9 reported a rate of 0.7% (1/138) for MOW HTO, and Georgoulis et al15 reported a rate of 0.4% (1/250) for LCW HTO. Most reported cases present as a symptomatic false aneurysm and require open vascular repair. We believe that knee flexion does not guarantee removal of the popliteal vessels from potential harm during surgery of the knee. Smith et al28 showed considerable variation in behavior of the popliteal vessels with knee flexion based on magnetic resonance imaging in supine patients. Further, Klecker et al16 reported a 2.1% prevalence of aberrant anterior popliteal artery, ventral to the popliteus muscle. We believe that a blunt Hohmann retractor placed directly along the posterior tibial cortex may offer the best protection against this catastrophic injury. There was no case of compartment syndrome. This includes the 16.1% of cases that were combined with primary or revision ACL reconstruction. Marti and Jakob20 reported the only case of compartment syndrome in a MOW HTO. As it was combined with ACL reconstruction, it was attributed to the accumulation of irrigation fluid. Deep infection occurred in 1.7% of cases. Previous studies (sample sizes between 35 and 190 patients) report rates between 0% and 3%.6,14,17,26,30,31,36,41 In studies with more than 100 patients, the rate ranges between 0.4% (1/ 262) and 2.7% (14/286).8,9,14,18,33 In a case-control study, Reischl et al25 reported an infection rate of 3.6% (4/106) for MOW HTOs. We believe that the key to infection treatment is hardware removal as soon as union is achieved. We had a recurrence in all cases with hardware kept in place. Spahn31 also commented on the treatment of infections with irrigation and debridement with gentamycin beads only. The rate of CRPS type 2 was 1.3%. No comparison is available in the literature. There was hardware failure with loss of correction in 1.0% of cases. These 3 patients received a nonlocking plate, had a fracture of the lateral hinge, and reported bearing weight on the limb earlier than instructed in the postoperative protocol. Loss of correction occurred when the plate or 2 screws broke on the same side of the plate. If only 1 screw broke (4.0%, 12/301), hip-to-ankle radiographs did not show any loss of correction. Stoffel et al32 reported that plates fail first by fracture of the lateral hinge. Rates of failure of nonlocking 4-hole plates have been reported to be between 3.6% (2/55) and 6.1% (3/49).24,31 In a review of the literature, Schroter et al26 stated that no such failures have been reported with the use of a locking plate. More recently, Floerkemeier et al14 reported a single case (0.2%; 1/386). Reported rates of isolated screw breakage were between 12.7% (7/55) and 20.8% (10/49).24,31 Several factors have been suggested to affect the occurrence of adverse events rates in MOW HTO, especially those related to delayed union and nonunion. These include

wedge size, the material used to fill the wedge, plate type, fracture of the lateral hinge, smoking, body mass index, and the postoperative weightbearing protocol.2,21,38 Investigating those proposed risk factors is complicated substantially by the low number of events and the likely possibility that they involve a combination of multiple factors. For example, the rates of delayed union and nonunion reported in the present sample are different for patients depending on smoking status, diabetes, and plate type, but the number of events in these subgroups is too low to enable confident statistical comparisons. We suggest that future investigations of the risk factors for adverse events in MOW HTO will require consistent definitions, should control for multiple factors that may interact with one another, and may benefit from using the classifications presently proposed. The present finding that patients experiencing class 2 and 3 adverse events had lower WOMAC scores preoperatively is intriguing in this regard and suggests that future studies may benefit from more closely examining the effect of preoperative comorbidities. The most important limitation in the present study is the retrospective review of data. Although adverse events were precisely defined before medical records were reviewed, some of the data in the medical records were reported by the representatives of the attending surgeons, who varied in their level of training. In an attempt to limit bias, a single orthopaedic surgeon not involved in the patient’s care (R.M.) directly evaluated the original radiographs (and CT scans, if ordered) and directly reviewed the laboratory results. However, it is possible that the reporting of adverse events, especially those that relied solely on the original reports, is affected by bias (resulting in under- or overestimates). The most severe adverse events that required additional or revision surgery and/or long-term medical care are less likely to be biased because those data were extracted from the administrative records. Additionally, we must acknowledge that although the overall sample size is still quite high, we did not have patient-reported data for all patients, and the number experiencing class 3 events is very low.

CONCLUSION The rate of severe adverse events in MOW HTO is low (7%). The present data suggest that severe adverse events are accompanied by lower patient-reported outcomes at 6 months yet similar outcomes by 12 and 24 months postoperatively. Additional surgery is required in 2.7% of cases and readmission in 4% of cases. The most common severe adverse event is nonunion (3.2%). The most common event requiring nonoperative treatment is delayed union (12%). The present classification system may be used in future research to help identify and control possible risk factors for these groups of adverse events.

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