Background: Knee arthroscopy is one of the most common surgical procedures worldwide and the number of arthroscopies has substantially increased in the last 30 years. Thus, our aim was to provide updated estimates on the risk of complications and compare it with the background risk in the general population. Methods: We identified patients aged 15–84 years with knee arthroscopy in the years 2005–2016 in southern Sweden. We calculated the risk of pyogenic arthritis, venous thromboembolism, and other typical complications within 30 days. As a reference cohort we included the regional population in the corresponding age interval. We estimated the relative and absolute risks of complications compared to the reference cohort using logistic regression adjusted for age, sex, and level of education. We also estimated the proportion of complications in the population explained by knee arthroscopy (population attributable fraction). Results: We identified 18,735 knee arthroscopy patients (mean age 39 years) and 1,171,084 reference subjects (mean age 46 years). The absolute risk of one or more complications was 1.1% after knee arthroscopy and 0.16% in references. The odds ratio of any complication after knee arthroscopy vs. the reference cohort was 9.4 (95% confidence interval [CI] 8.1, 10.9) with an absolute risk difference of 1.4% (1.1, 1.6%). The relative risk (95% CI) for pyogenic arthritis was 115 (75, 174), venous thromboembolism 6.8 (5.1, 9.1), and other complications 7.7 (6.3, 9.5). The population attributable fraction for pyogenic arthritis was 5%. Conclusions: The absolute risks of complications associated with knee arthroscopy remain small at about 1%. Still, 5% of all pyogenic knee arthritis cases in adults are attributable to knee arthroscopy, thus risks with knee arthroscopy should be carefully considered in the choice of treatment. Keywords: Knee arthroscopy, Complications, Pyogenic arthritis, Venous thromboembolism, Epidemiology Background Knee arthroscopy is widely acknowledged to be a safe Knee arthroscopy is one of the most common surgical procedure [4, 6]. Still, there are also known serious com- procedures worldwide  and the number of arthrosco- plications such as joint infection, deep vein thrombosis, pies has substantially increased in the last 30 years . pulmonary embolism, and there are even deaths reported The patients are often younger or middle-aged physically [5, 7, 8]. For more complex arthroscopic procedures that active individuals. Annually, there are about one million involve ligament reconstruction, the risk of complication such procedures performed in the United States and in has been reported to be the highest followed by meniscal Sweden (population 9.5 million) the corresponding repair, chondroplasty, and meniscectomy [5, 7]. However, number is about 35,000 [2, 3]. The most common there is a substantial variation in the reported absolute procedures are meniscectomy, meniscal repair, and risks for complications – pyogenic arthritis between 0.08% cruciate ligament reconstruction [2, 4, 5].  and 0.42% , deep vein thrombosis between 0.12%  and 41% , and pulmonary embolism between 0.03%  and 0.11% . * Correspondence: email@example.com Department of Clinical Sciences Lund, Orthopaedics, Clinical Epidemiology Because knee arthroscopy has become high volume Unit, Faculty of Medicine, Lund University, Remissgatan 4, 221 85 Lund, surgery, and the procedure is also more and more fre- Sweden 2 quently performed in older patients, it is of importance Clinical Epidemiology Research & Training Unit, Boston University School of Medicine, Boston, MA, USA © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Friberger Pajalic et al. BMC Musculoskeletal Disorders (2018) 19:179 Page 2 of 7 to gain updated information on safety of the procedure. Table 1 The different knee arthroscopic procedures included in the study Further, there are as of yet no studies comparing the risks of complications to the underlying risks for these Code Knee arthroscopic procedure events in the background population. Such comparisons NGA01 Exploration of soft tissue of knee or lower leg; arthroscopic are needed to get a better understanding of the share of complications attributable to the arthroscopy. Hence, NGA11 Exploration of knee joint; arthroscopic the two main purposes of our study were to: 1) deter- NGA21 Biopsy of soft tissue of knee or lower leg; arthroscopic mine updated risks for a number of pre-specified com- plications within 30 days after knee arthroscopy in NGA31 Biopsy of bone of knee or lower leg; arthroscopic absolute terms as well in relative terms compared to the background population, and 2) to estimate the propor- NGD01 Total excision of meniscus; arthroscopic tion of the total number of complications in the society NGD11 Partial excision of meniscus; arthroscopic explained by knee arthroscopies. NGD21 Reinsertion of meniscus; arthroscopic NGD91 Other operation on meniscus; arthroscopic Methods NGE01 Incision or suture of joint capsule of knee; Data sources arthroscopic The Skåne Healthcare Register (SHR) NGE11 Transcision or excision of ligament of knee; In Skåne region, the most southern part of Sweden, infor- arthroscopic mation about all healthcare visits (primary and secondary NGE21 Suture or replantation of ligament of knee; outpatient care and hospitalizations) is collected in the arthroscopic Skåne Healthcare Register (SHR). Doctors register the NGE31 Transposition of ligament of knee; arthroscopic diagnostic codes according to the International Classifi- NGE41 Reconstruction of ligament of knee without cation of Diseases (ICD)-10 system. Codes for surgical foreign object; arthroscopic procedures are registered according to the classification NGE51 Reconstruction of ligament of knee with of healthcare procedures . In addition, information foreign object; arthroscopic about the type and level of the health care and the date NGE91 Other surgery on capsule or ligament of knee; of the visit is included. arthroscopic NGF01 Total synovectomy of knee; arthroscopic The Swedish population register NGF11 Partial synovectomy of knee; arthroscopic The Swedish population register contains information NGF21 Fixation of fragment of surface of knee; about deaths, gender, and residential address for all resi- arthroscopic dents. This register is continuously updated by the Swedish NGF31 Partial excision of joint cartilage of knee; Tax Agency. arthroscopic NGF91 Other operation on synovia or joint surface Study cohorts and exposure of knee; arthroscopic For both patients having had knee arthroscopy and the NGH31 Freeing of adhesions of knee joint; arthroscopic reference population we used the same principal inclu- NGH51 Excision of intraarticular exostosis or osteophyte sion criteria: we required all persons to be i) aged 15 to of knee; arthroscopic 84 years and ii) to be residents in the Skåne region at NGH71 Surgery for habitual dislocation of knee joint; the time of the index visit. arthroscopic In our exposed cohort, the exposure of interest was a NGH91 Other operation of knee joint; arthroscopic knee arthroscopy (index visit), defined by the surgical Surgical codes according to “Klassifikation av. vårdåtgärder” (KVÅ) [in Swedish] codes in the SHR between February 1st 2005 and November 30th 2016. In brief, the selected surgical knee procedures included exploration of the joint, resection 30th 2016. We randomly selected an index date from a of the meniscus, meniscus repair, ligament reconstruc- calendar year with at least one healthcare visit (Fig. 1). tion, and synovectomy (Table 1). We included each We did not include hospitalizations (i.e. stays at person only once, at the time of their first knee arthros- hospital overnight) because all the diagnostic codes are copy during the inclusion period. registered once, at the time of discharge. Thus, the true In our unexposed cohort, i.e., background population temporal order of diagnoses is unknown, e.g., if pyogenic not having knee arthroscopy, we included all persons in arthritis developed due to other reasons and was the in- the Skåne region with at least one healthcare visit in out- dication to perform the arthroscopic lavage there would patient care or day surgery (excl. Cases of knee arthros- have been a risk for reverse causality and thus inflated copy) between the February 1st 2005 and November estimates of risk. Of all knee arthroscopies 6.5% were Friberger Pajalic et al. BMC Musculoskeletal Disorders (2018) 19:179 Page 3 of 7 Fig. 1 Flow chart of the study cohorts made during hospitalisations and were thus not included to e.g. the drilling of bone tunnels and the typically long in the present study. duration of the procedure [5, 7]. For the purpose of a sensitivity analysis we created an Outcome alternative reference group only containing patients who The outcome was having at least one of our pre-specified had consulted for a knee problem but having no knee complications diagnosed in specialist care within 30 days arthroscopy. We used the same study period and age after the index visit (Table 2). criterion as detailed above. However, we sampled reference The main diagnoses of interest were pyogenic arthritis subjects, who all have had at least one clinic visit to a (ICD-10 code M00), venous thrombotic event (VTE, in- physician, with at least one primary diagnostic code for a cluding thrombophlebitis, ICD-10 code I80 and pulmonary knee problem which may have represented an indication embolism, ICD-10 code I26), complications of procedures for knee arthroscopy, but the patient did not have knee (ICD-10 code T81), local infections (ICD-10 code L08), surgery. The included diagnostic codes were: Meniscus sepsis (ICD-10 code A41) and mistakes done during the tear, old (M23.2), Chronic instability of knee (M23.5), surgery (ICD-10 codes Y60, 61, 69 and 71). Primarily, the Unilateral primary osteoarthritis of knee (M17.1), Trau- outcome was having any of the aforementioned com- matic tear of meniscus (S83.2), Sprain and strain involving plications. Additionally, we performed analyses with cruciate ligament of knee (S83.5), Other post-traumatic thespecificcomplicationofpyogenicarthritis, VTE, gonarthrosis (M17.3), Tear of articular cartilage of knee, and other complications, respectively. current (S83.3), Other secondary gonarthrosis (M17.5), Injury to multiple structures of knee (S83.7), Patellofemoral Data analysis strategy disorders (M22.2), Dislocation of patella (S83.0), Other To avoid the risk of bias due to reverse causality we only internal derangements of knee (M23.8), and Recurrent included complications registered from day 1 to day 30 dislocation of patella (M22.0). In this sensitivity analysis we after the index visit. The number of complications regis- also only included arthroscopies with one the above tered on day 0 was only 15 in the arthroscopy patients diagnoses registered as main diagnosis. In the analysis, the and 84 in the reference cohort. In addition, we excluded date of the healthcare visit (reference subjects) or arthros- all subjects (both in the arthroscopy group and the refer- copy was the index visit. A reference subject was excluded ence group) who had any of the aforementioned compli- if he/she had any knee surgery (KVÅ code starting with cations up to 30 days before the index visit. “NG”) within 30 days before the index visit as well as a knee We also evaluated the risk for complications after cruci- surgery other than knee arthroscopy 0 to 30 days after the ate ligament reconstructions (code NGE41 group within index visit. the KVÅ system, Table 1) as compared to the other arthros- copies. Additionally, we estimated the risk of complications Statistical analysis after arthroscopy compared to the background population, We present descriptive data for the study sample as means after excluding cruciate ligament reconstruction from the and standard deviations (SD) for the continuous variables arthroscopy group. The rationale with these two latter and as counts and percentages for categorical variables. analyses was that cruciate ligament reconstructions have We estimated the odds ratios (ORs) and risk differences been reported to have a higher risk for complications due using a logistic regression model. We present both crude Friberger Pajalic et al. BMC Musculoskeletal Disorders (2018) 19:179 Page 4 of 7 Table 2 ICD-10 codes for the complications included in the categories (primary school, high school, higher education study < 3 years, and higher education ≥3 years). Individual-level ICD-10 code Complication (Adverse event, Complication data on level of education were retrieved from Statistics outcome). Specialist in and group Sweden (provider of official statistics in Sweden) by register out-patient care cross-linking. Please note, due to low prevalence of the I80 Phlebitis and thrombophlebitis VTE outcome the ORs can be interpreted as risk ratios . I26 Pulmonary embolism The risk difference was calculated from the adjusted M00 Pyogenic arthritis Pyogenic logistic regression model as the difference of the mean arthritis probabilities predicted by the model, as implemented A41 Other sepsis Other in the command adjrr in Stata . The population L08 Other local infections of skin and attributable fraction was estimated from the adjusted subcutaneous tissue risk ratios and the prevalence of exposure among cases T81 Complications of procedures, not (i.e. those with outcome) as prevalence_among_cases*((ri- elsewhere classified sk_ratio-1)/risk_ratio). All estimates are presented with its T81.0 Bleeding and hematoma as 95% confidence intervals (95% CI). complication due to surgical and medical procedures, not elsewhere Results classified Demographics T81.1 Post procedural shock Out of our 1,171,084 included subjects, 18,735 (1.6%) had T81.2 Accidental puncture or damage undergone knee arthroscopy. Mean age for the knee arth- during surgical and medical procedure, not elsewhere classified roscopy cohort and the reference cohort was 39.4 years (SD 15.4) and 45.5 years (SD 20.3), respectively (Table 3). T81.3 Disruption of wound, not elsewhere classified T81.4 Infection following a procedure Indications for knee arthroscopy and type of procedures The four most common diagnoses registered as the pri- T81.5 Complications of foreign body accidentally left in body following mary diagnosis for the knee arthroscopy were: meniscus procedure tear, old (ICD-10 M23.2, 35%), chronic instability of knee T81.6 Acute reaction to foreign substance (M23.5, 19%), unilateral primary osteoarthritis of knee accidentally left during a procedure (M17.1, 11%) and traumatic tear of meniscus (S83.2, 6%). T81.7 Vascular complications following a The three most frequent arthroscopic procedures were procedure, not elsewhere classified partial excision of meniscus (44%), reconstruction of T81.8 Other complications of procedures, ligament of knee with/without foreign object (17%) and not elsewhere classified exploration of knee joint (diagnostic arthroscopy) (16%). T81.9 Unspecified complication of procedure Risk of complications Y60 Unintentional cut, puncture, We found that the risk for any of our pre-specified com- perforation or haemorrhage during surgical and medical care plications in the knee arthroscopy cohort was 1.1% (210 persons) and in the reference cohort 0.16% (1818 per- Y61 Foreign object accidentally left in body during surgical and medical sons) during the follow-up period of 30 days. The most care common complications were: complications of proce- Y69 Unspecified misadventure during dures not classified elsewhere (33% of all complications), surgical and medical care thrombophlebitis (24%) and pulmonary embolism (14%). Y79 Orthopaedic devices associated The crude OR of having any complication after a knee with adverse incidents arthroscopy compared to the reference cohort was 7.3 Death while it was 9.4 (95% CI 8.1, 10.9) adjusted for age, gen- M00 in joints other than the knee were not included der, and level of education (Table 4). The corresponding Data on death date were retrieved from the Population Register risk difference was 1.4%. Among the adjusted ORs for the specific complications, pyogenic arthritis yielded by results (i.e. from an unadjusted model) and results from a far the highest OR; 115 (95% CI 75, 174). The OR for model that was adjusted for 3 pre-specified confounders, VTE was 6.8 and for having other complications the OR i.e., characteristics that we considered to be associated with was 7.7 (Table 4). both knee surgery and the risk of complications [13, 14]. In our sensitivity analysis (the comparison against knee These were age (included as continuous variable), sex patients but without knee arthroscopy) the OR of any (male or female), and level of education with four aforementioned complication (knee arthroscopy vs. the Friberger Pajalic et al. BMC Musculoskeletal Disorders (2018) 19:179 Page 5 of 7 Table 3 The description of the study cohorts Main analysis Sensitivity analysis Arthroscopy patients, References (general population), Arthroscopy patients, References (knee patients), n = 18,735 n = 1,171,084 n = 15,528 n = 44,804 Age, mean (SD) years 39.4 (15.4) 45.5 (20.3) 40.1 (15.4) 49.8 (19.5) Women, n (%) 7398 (39.5) 604,583 (51.6) 5900 (38) 22,721 (51) Level of education (years), n (%) Primary school (up to 9) 3824 (21) 282,043 (27) 3114 (21) 11,611 (27) High school (10–12) 8984 (50) 436,465 (41) 7555 (50) 18,863 (44) Higher education (13–14) 2419 (13) 141,418 (13) 2057 (14) 5277 (12) Higher education (15+) 2855 (16) 204,085 (19) 2407 (16) 6803 (16) references) was 4.9 in the adjusted model with an abso- and more frequently performed also in middle-aged lute risk difference of 1.0%. The corresponding ORs for patients. Also the surgical routines may have improved pyogenic arthritis, VTE, and other complications were which calls for a need of new estimates of risks associated 10.3, 2.3, and 7.9, respectively (Table 4). with the procedure. Thus, we used comprehensive Swedish The population attributable fraction was 5.1% for pyo- health care data over a 12-year time period to evaluate the genic arthritis, 0.2% for VTE, and 0.3% for the other risks for the most frequent complications during a 30-day complications (i.e., about 5% of all pyogenic knee arth- follow-up period. ritis cases were attributable to the knee arthroscopy). It has been previously reported that knee arthroscopy The OR for complications after ligament reconstruction is associated with complications such as pyogenic arth- surgery compared to other arthroscopies adjusted for age, ritis and VTE [4, 7, 10, 17]. However, the estimated sex, and level of education was 4.1 (95% CI 2.9, 5.8). The over-all risk of complications varies from 0.27 to 8.2% OR for complications after a knee arthroscopy excluding with the risk being higher after more complex and ligament reconstruction compared to the reference popu- longer lasting arthroscopies . Reigstad et al.  pre- lation was 6.6 (95% CI 5.5, 8.0) with a corresponding risk sented an over-all risk of 5%, but pain and swelling were difference of 0.92% (95% CI 0.73, 1.10%). included in this study which represented almost 50% of all complications. In our study, we focused on the more Discussion severe complications which explain why our estimate is The absolute risk of having a complication such as pyo- lower. Sherman et al.  presented the over-all risk of genic arthritis, VTE, or other surgical complications complications after knee arthroscopy to be about 8%. In after knee arthroscopy was 1.1%. The increase in risk of the study the authors classified complications into minor these conditions as compared to the general population and major. Several of the major complications were was about 9-fold, and compared to other patients with a more similar to the ones selected in our study such as knee condition (but no knee arthroscopy) about 5-fold. infections, cardiovascular, neurological and instrument There is a need for updated estimates on the risks associ- failure. Further, the relatively high over all risk could also ated with knee arthroscopy as there has been an increase potentially partly be explained by the study being in number of procedures, but the procedure is also more 20 years old and that arthroscopic surgery has developed Table 4 Odds ratios (OR) and risk differences (RD) with 95% confidence intervals (CI) for complications after knee arthroscopy compared to reference persons. Results are from a logistic regression model, crude (unadjusted) or adjusted for age, sex and education level a b Main analysis Sensitivity analysis Crude Adjusted Crude Adjusted OR (95%CI) OR (95%CI) RD (95%CI), % OR (95%CI) OR (95%CI) RD (95%CI), % Any complication 7.3 (6.3,8.4) 9.4 (8.1, 10.9) 1.35 (1.14, 1.56) 5.0 (3.9, 6.4) 4.9 (3.8, 6.4) 0.95 (0.76, 1.15) Pyogenic arthritis 127 (85, 191) 115 (75, 174) 0.37 (0.27, 0.47) 13.8 (7.6, 25.1) 10.3 (5.6, 19.0) 0.30 (0.21, 0.39) VTE 4.8 (3.6, 6.4) 6.8 (5.1, 9.1) 0.37 (0.25, 0.49) 2.0 (1.4, 3.0) 2.3 (1.5, 3.5) 0.17 (0.07, 1.30) Other 6.4 (5.2, 7.8) 7.7 (6.3, 9.5) 0.61 (0.47, 0.60) 7.8 (5.3, 11.6) 7.9 (5.2, 11.9) 0.54 (0.40, 0.68) knee arthroscopy compared to general population knee arthroscopy compared to persons consulting for knee problems venous thrombotic event Friberger Pajalic et al. BMC Musculoskeletal Disorders (2018) 19:179 Page 6 of 7 since with better equipment and better surgical routines specialist care, where the validity of the diagnostic codes at large. In the rest of the existing literature, which is (such as pyogenic arthritis or VTE) is expected to be somewhat more recent, the reported risk of complica- higher than in primary care. We did not include the tions is reported to be below 5%. In one study from complications registered on day 0 to minimize the risk of Japan  the investigators report a very low over all risk reverse causality (pyogenic arthritis). Thus, the true abso- for complications of merely 0.27% in a relatively small lute risks may rather have been underestimated, but is not cohort. In larger cohort studies the risk is typically re- expected to substantially influence the relative risks or the ported to be between 0.64 and 4.7% [7, 18, 19], which is population attributable fraction. Another important limi- more in line with our findings. The follow-up period in tation is a potential for bias from unmeasured con- most studies has typically been up to some 30 days [5, 7, founding (e.g. smoking and body mass index) and bias 9, 18, 19] with the exception of a few studies with 90 days by indication – i.e. the systematic differences between of follow-up [8, 11, 18]. In our study, we chose 30 days persons undergoing arthroscopic surgery and the under- of follow-up because the most common complications, lying population. We aimed to take the latter into account such as infections and VTE, attributed to the surgery are in our sensitivity analysis, where only persons with a likely to be diagnosed within that time frame. potential indication for a knee arthroscopy were included Partial meniscectomy was the most common proced- which yielded similar results, except for pyogenic arthritis ure in our study (44% of all knee arthroscopies) which is where we noted a substantial attenuation of the risk in line with existing literature [2, 5, 8, 11]. Although estimate. Unfortunately, there is no data on body length arthroscopic partial meniscectomy carries a smaller risk and body weight available in the register. for complications as compared to more complex surgery, such as ligament reconstruction, the reported Conclusions overall risk is still not negligible . This is in line with To conclude, although the absolute risk of complication our results, where even after excluding the ligament of 1.1% is small, knee arthroscopy should not be consid- reconstruction from the arthroscopy group, the relative ered a completely benign intervention. In particular as it risk of a complication was still almost 7-fold as com- is high volume surgery at most knee surgery clinics. For pared to the general population. example, the procedure may be responsible for 5% of cases Since knee arthroscopy is the most common ortho- of pyogenic arthritis of the knee. Thus, it remains import- paedic procedure this results quite many patients experi- ant to communicate risks with the patient, and consider encing a complication even though the absolute risk is non-surgical treatment options when appropriate. low. For example, our data suggests that 5% of all cases of Abbreviations pyogenic arthritis are caused by knee arthroscopy. Thus, CI: confidence interval; ICD: International Classification of Diseases; OR: odds indications for arthroscopic surgery need to be scrutinized ratio; SHR: Skåne healthcare register; VTE: venous thrombotic event as proper evidence of efficacy above non-surgical manage- Funding ment is sometimes lacking and compliance to new guide- The study was supported by the Swedish Research Council, Kock Foundations, lines still needs to be verified, e.g. for degenerative The Swedish Rheumatology Association, Österlund Foundation, the Faculty of Medicine, Lund University, Sweden, and Governmental Funding of Clinical meniscal lesions and knee osteoarthritis [20, 21]. Research within National Health Service (ALF). The funders had no role in the Our study is based on a large study population from an design of the study, data collection, analysis, interpretation of data, in writing entire Swedish region without selection bias. The original the manuscript, or decision to publish. data is all prospectively registered in the electronic medical Availability of data and materials records, i.e., free of any potential recall bias from both the The datasets analyzed during the current study are not publicly available due patient and doctor. We estimated not only the absolute to Ethical Review Board decision and safety and data integrity reasons stipulated under the Swedish Patient Data Act (Patientdatalagen 2008:355) risk for complications after knee arthroscopy but also the regarding sensitive personal data. relative risk compared to the background population. Still, we would like to acknowledge some important limitations: Authors’ contributions KFP participated in study design, did results interpretation and drafted the As in all register-based research, there is a risk of misclassi- first version of manuscript. AT did the study design, data acquisition, lead the fication of procedures and diagnostic codes which may lead statistical analyses, results interpretation, and participated in manuscript to biased results. However, the validity of the diagnostic revision. ME conceived the study and participated in study design, data acquisition, results interpretation, and lead the manuscript revision. All codes in SHR and Swedish health care registers in general authors have approved the final version of manuscript for submission. has been reported to be high [22–24]. We would like to emphasize that these registers are not just administrative Ethics approval and consent to participate data per se, as in many other countries, but diagnoses are The study was approved by the Lund University ethical review board (Dnr 2014–276). The need for individual informed consent was waived. actually set by the doctors’ themselves and data is drawn directly from the patients’ electronic medical records. 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