minimally invasive surgery total knee arthroplasty ( mis - tka ) requires downsizing or modification of surgical equipment due to the small field of view ; medial / lateral or proximal / distal movement of a retractor to secure an adequate field of view ; extension / flexion or medial / lateral rotation of the knee joint during surgery ; and , above all , continuous development of surgical techniques and cooperation of the members of a surgical team1,2,3,4,5 ) . in addition , the use of an implant designed specifically for mis can improve the efficiency of the surgery . the mini - keel modular tibial implant ( nexgen mis tibial component ; zimmer inc . , warsaw , in , usa ) is one of those inventions that can be inserted into the knee in flexed position without anterior dislocation of the knee joint due to the presence of a short keel in the tibial component ( fig . 1)6 ) . the purpose of this study was to evaluate the efficacy and longevity of the mini - keel modular tibial implant in mis - tka based on a retrospective review of clinical and radiological findings . a total of 375 tkas were performed by the same surgeon at our institution between november 2005 and october 2006 . of those , 361 tkas were performed via minimally invasive approach in 254 patients , and surgical outcomes and intraoperative complications were investigated among these patients . clinical and radiological assessments were performed in patients who were available for more than 5-year follow - up . in all cases , the nexgen legacy posterior stabilized flex fixed bearing ( lps flex fixed , zimmer inc . ) and the nexgen mis tibial component were used . the mean age of the patients was 68.8 + 5.6 years ( range , 53 to 80 years ) . the mean body mass index was 273.7 kg / m ( range , 20.6 to 36.8 kg / m ) . the indications for surgery were degenerative arthritis in 165 cases , rheumatoid arthritis in 2 cases , and osteonecrosis in 1 case . preoperatively , the mean range of motion ( rom ) was 125.217.7 ( range , 70 to 145 ) , the mean knee score was 65.07.6 points ( range , 22 to 82 points ) , and the mean femorotibial angle was 4.05.6 varus ( range , 21 varus to 2 valgus ) . clinical and radiological assessments were conducted at the last follow - up in 168 cases that were available for more than 5-year follow - up on an outpatient basis . one hundred and forty- one cases that were available for a phone interview only in january 2013 were excluded from the clinical evaluation and so were the 79 cases ( due to death in 23 , nursing home admission in 10 , intertrochanteric fracture in 4 , distal femur fracture in 3 , transtibial amputation in 1 , infection - related revision in 1 , immigration to a foreign country in 1 , and loss of contact in 36 cases ) . a curved skin incision was made from 5 mm above the medial patella to the superior aspect of the medial tibial tuberosity . in cases where severe skin retraction was observed or there was difficulty with the operation or implant insertion , the skin incision was extended gradually proximally or distally to avoid excessive tension on the skin . then , the joint was excised using the mini - midvastus approach7,8 ) : a 1.5 cm proximal incision was started superomedial to the patella in parallel to the vastus medialis muscle fibers and extended along the medial patella to 3 cm distal to the knee joint surface . the patella was resurfaced in all cases . a 6 valgus intramedullary alignment rod attached to a distal cutting guide was inserted into the center of the knee joint where the anatomical axis of the femur passes though . with the distal cutting guide pin fixed on the medial aspect of the medial femoral condyle , a proximal tibial cut was performed using an extramedullary alignment rod as in conventional tka . using the cutting guide , the osteotomy was initiated on the anteromedial aspect of the tibia and extended to involve approximately more than 80% of the proximal tibia except for some posterolateral and lateral areas . due to the limited field of view the rotational alignment of the femur was determined in reference to the posterior condylar axis . at the same time , the appearance of the ' grand - piano sign'9 ) on the anterior resected surface of the femur was confirmed before resection . with the tibial component temporarily press - fitted , a 45 mm drop - down stem extension was additionally inserted to increase the contact surface between the tibial component and the tibia and fixation strength . for the 3 postoperative days , a deep vein thrombosis ( dvt ) foot pump was used to prevent dvt without administration of thrombolytic agents . within 2 postoperative days , ambulation was permitted depending on the patient 's condition . on the postoperative clinical evaluation , the hospital for special surgery ( hss ) score , knee society score ( kss)10 ) , and rom were assessed at the last follow - up in the 168 cases that were available for more than 5-year follow - up . on the radiological evaluation , the femorotibial angle , tibial component alignment angle , and tibial component posterior inclination were assessed on the standing anteroposterior and lateral views taken at the last follow - up according to the knee society total knee arthroplasty roentgenogrphic evaluation11 ) . in addition , implant location and fixation status were assessed . to assess the implant fixation status , the bone - cement or cement - implant contact area was divided into 7 zones on the lateral view for the femoral component , whereas into 7 zones on the anteroposterior view and 3 zones on the lateral view for the tibial component . the width of radiolucent lines on 5 different zones on the skyline view was measured to assess the stability of the patellar component . presence of a 2-mm - wide radiolucent line , implant subsidence , or change in the alignment was considered as a sign of radiological loosening . . 12.0 ( spss inc . , chicago , il , usa ) was used for statistical analysis . preoperative and postoperative clinical findings were compared using a t - test with statistical significance set at p<0.05 . a total of 375 tkas were performed by the same surgeon at our institution between november 2005 and october 2006 . of those , 361 tkas were performed via minimally invasive approach in 254 patients , and surgical outcomes and intraoperative complications were investigated among these patients . clinical and radiological assessments were performed in patients who were available for more than 5-year follow - up . in all cases , the nexgen legacy posterior stabilized flex fixed bearing ( lps flex fixed , zimmer inc . ) and the nexgen mis tibial component were used . the mean age of the patients was 68.8 + 5.6 years ( range , 53 to 80 years ) . the mean body mass index was 273.7 kg / m ( range , 20.6 to 36.8 kg / m ) . the indications for surgery were degenerative arthritis in 165 cases , rheumatoid arthritis in 2 cases , and osteonecrosis in 1 case . preoperatively , the mean range of motion ( rom ) was 125.217.7 ( range , 70 to 145 ) , the mean knee score was 65.07.6 points ( range , 22 to 82 points ) , and the mean femorotibial angle was 4.05.6 varus ( range , 21 varus to 2 valgus ) . clinical and radiological assessments were conducted at the last follow - up in 168 cases that were available for more than 5-year follow - up on an outpatient basis . one hundred and forty- one cases that were available for a phone interview only in january 2013 were excluded from the clinical evaluation and so were the 79 cases ( due to death in 23 , nursing home admission in 10 , intertrochanteric fracture in 4 , distal femur fracture in 3 , transtibial amputation in 1 , infection - related revision in 1 , immigration to a foreign country in 1 , and loss of contact in 36 cases ) . a curved skin incision was made from 5 mm above the medial patella to the superior aspect of the medial tibial tuberosity . in cases where severe skin retraction was observed or there was difficulty with the operation or implant insertion , the skin incision was extended gradually proximally or distally to avoid excessive tension on the skin . then , the joint was excised using the mini - midvastus approach7,8 ) : a 1.5 cm proximal incision was started superomedial to the patella in parallel to the vastus medialis muscle fibers and extended along the medial patella to 3 cm distal to the knee joint surface . the patella was resurfaced in all cases . a 6 valgus intramedullary alignment rod attached to a distal cutting guide was inserted into the center of the knee joint where the anatomical axis of the femur passes though . with the distal cutting guide pin fixed on the medial aspect of the medial femoral condyle , a proximal tibial cut was performed using an extramedullary alignment rod as in conventional tka . using the cutting guide , the osteotomy was initiated on the anteromedial aspect of the tibia and extended to involve approximately more than 80% of the proximal tibia except for some posterolateral and lateral areas . due to the limited field of view the rotational alignment of the femur was determined in reference to the posterior condylar axis . at the same time , the appearance of the ' grand - piano sign'9 ) on the anterior resected surface of the femur was confirmed before resection . with the tibial component temporarily press - fitted , a 45 mm drop - down stem extension was additionally inserted to increase the contact surface between the tibial component and the tibia and fixation strength . for the 3 postoperative days , a deep vein thrombosis ( dvt ) foot pump was used to prevent dvt without administration of thrombolytic agents . on the postoperative clinical evaluation , the hospital for special surgery ( hss ) score , knee society score ( kss)10 ) , and rom were assessed at the last follow - up in the 168 cases that were available for more than 5-year follow - up . on the radiological evaluation , the femorotibial angle , tibial component alignment angle , and tibial component posterior inclination were assessed on the standing anteroposterior and lateral views taken at the last follow - up according to the knee society total knee arthroplasty roentgenogrphic evaluation11 ) . in addition , implant location and fixation status were assessed . to assess the implant fixation status , the bone - cement or cement - implant contact area was divided into 7 zones on the lateral view for the femoral component , whereas into 7 zones on the anteroposterior view and 3 zones on the lateral view for the tibial component . the width of radiolucent lines on 5 different zones on the skyline view was measured to assess the stability of the patellar component . presence of a 2-mm - wide radiolucent line , implant subsidence , or change in the alignment was considered as a sign of radiological loosening . spss ver . 12.0 ( spss inc . , chicago , il , usa ) was used for statistical analysis . preoperative and postoperative clinical findings were compared using a t - test with statistical significance set at p<0.05 . the mean follow - up period was 6 years and 1 month ( minimum , 5 years ; maximum , 7 years and 7 months ) . the mean postoperative femorotibial angle was 5.21.7 valgus ( range , 2.6 to 8.1 ) and the mean tibial component alignment angle was 90.21.6 varus ( range , 86.5 to 93.4 ) . the mean tibial component posterior inclination was 4.82.1 ( range , 1.9 to 8.1 ) . the femorotibial alignment angle was 63 valgus in 340 of the 361 cases ( 94.0% ) and the tibial component alignment angle was 03 varus in 347 cases ( 96.1% ) ( table 1 ) . the mean rom was increased from 125.217.7 ( range , 70 to 145 ) preoperatively to 134.312.4 ( range , 85 to 145 ) postoperatively . the mean postoperative kss was 94.55.9 points ( range , 68 to 100 points ) , function score was 84.815.6 points ( range , 54 to 100 points ) , and hss was 92.77.0 points ( range , 62 to 100 points ) , all of which were significantly improved after surgery ( table 2 ) . revision was not necessary in all cases based on the last follow - up radiological assessment which showed no evidence of femoral / tibial component alignment change , subsidence , or loosening . radiolucency was observed in more than one zone in the bone - implant interface in 22 cases ( 13.1% ) at the last follow - up : in the femur and tibia both in 4 cases , in the femur only in 10 cases , in the tibia only in 7 cases , and in the patella only in 1 case . the components demonstrated stable fixation without signs of alignment changes or loosening in all cases . the radiolucency was observed in 1 zone in 15 cases , in 2 zones in 4 cases , and in more than 3 zones in 3 cases . in the femur , it was located in zone 1 in 8 cases , in zone 2 in 1 case , and in zone 4 in 8 cases . in the tibia , it was observed in zone 1 in 9 cases , in zone 2 in 3 cases , and in zones 4,5 , and 7 in 1 case each on the anteroposterior view whereas in zone 3 in 2 cases on the lateral view . in the patella , it was found in zone 1 in 1 case ( table 3 ) . the mean survival rate per kaplan - meier analysis was 99.4% at 6.1 years ( fig . revision was required due to component loosening in one of the 168 cases that were followed for more than 5 years . the mean follow - up period was 6 years and 1 month ( minimum , 5 years ; maximum , 7 years and 7 months ) . the mean postoperative femorotibial angle was 5.21.7 valgus ( range , 2.6 to 8.1 ) and the mean tibial component alignment angle was 90.21.6 varus ( range , 86.5 to 93.4 ) . the mean tibial component posterior inclination was 4.82.1 ( range , 1.9 to 8.1 ) . the femorotibial alignment angle was 63 valgus in 340 of the 361 cases ( 94.0% ) and the tibial component alignment angle was 03 varus in 347 cases ( 96.1% ) ( table 1 ) . the mean rom was increased from 125.217.7 ( range , 70 to 145 ) preoperatively to 134.312.4 ( range , 85 to 145 ) postoperatively . the mean postoperative kss was 94.55.9 points ( range , 68 to 100 points ) , function score was 84.815.6 points ( range , 54 to 100 points ) , and hss was 92.77.0 points ( range , 62 to 100 points ) , all of which were significantly improved after surgery ( table 2 ) . revision was not necessary in all cases based on the last follow - up radiological assessment which showed no evidence of femoral / tibial component alignment change , subsidence , or loosening . radiolucency was observed in more than one zone in the bone - implant interface in 22 cases ( 13.1% ) at the last follow - up : in the femur and tibia both in 4 cases , in the femur only in 10 cases , in the tibia only in 7 cases , and in the patella only in 1 case . the components demonstrated stable fixation without signs of alignment changes or loosening in all cases . the radiolucency was observed in 1 zone in 15 cases , in 2 zones in 4 cases , and in more than 3 zones in 3 cases . in the femur , it was located in zone 1 in 8 cases , in zone 2 in 1 case , and in zone 4 in 8 cases . in the tibia , it was observed in zone 1 in 9 cases , in zone 2 in 3 cases , and in zones 4,5 , and 7 in 1 case each on the anteroposterior view whereas in zone 3 in 2 cases on the lateral view . in the patella , it was found in zone 1 in 1 case ( table 3 ) . the mean survival rate per kaplan - meier analysis was 99.4% at 6.1 years ( fig . revision was required due to component loosening in one of the 168 cases that were followed for more than 5 years . determining whether mis is an appropriate treatment option in tka requires assessments on its conformity to standard operating principles , such as flexion / extension gap balancing , restoration of proper alignment , prevention of intraoperative complications , and achievement of expected surgical outcome1,2,7,12,13,14,15 ) . in the current study , the mis - tka demonstrated relatively high accuracy : the tibial component alignment angle was 03 varus in 347 of the 361 cases ( 96.1% ) and the femorotibial angle was 63 valgus in 340 cases ( 94.0% ) . this can be attributed to the proper placement of the cutting alignment guide during surgery in spite of the limited field of view , considering that femoral and tibial alignments are dictated by the intramedullary / extramedullary alignment guide . during surgery , we expected the side cutting technique for distal femoral cut would result in minimal soft tissue damage and verified firm attachment of the distal cutting guide to the 6 valgus intramedullary alignment rod for precise osteotomy in the medial condyle of the distal femur . for lateral femoral resection , this intramedullary alignment rod should be removed : thus , the remaining distal cutting guide on the medial aspect of the medial femoral condyle can be displaced by soft tissue during the procedure . in addition , the osteotomy should be carried out in the absence of proper field of view , rendering it difficult to conduct precise resection . to overcome this obstacle , we removed the distal cutting guide as well and used a free hand technique for lateral femoral condyle resection by referring to the resected surface of the medial femoral condyle . with experience , mis - tka can be performed through a small incision without applying excessive tension on the surrounding soft tissue : however , implant insertion can still be challenging if the field of view is extremely limited1,3,4,14 ) . in order to insert conventional tibial components that have long stems , the tibial resection surface should be sufficiently anteriorly dislocated relative to the femoral resection surface with the knee in flexion . this may be not feasible if the incision is small , result in complications caused by excessive tension on the adjacent tissue , or require additional soft tissue resection . the modular tibial implant ( nexgen mis precoat stemmed tibial plate & drop - down stem extensions ) was designed to overcome these disadvantages by allowing implant insertion in knee flexion without anterior dislocation of the tibia through a small field of view . the modular tibial implant is made of titanium alloy and composed of a mini - keel tibial plate and a 45 mm- or 75 mm - long modular stem ( drop - down stem extension ) . the short keel length ( range , 17 to 19 mm ) in the tibial plate facilitates insertion into a flexed knee without anterior dislocation of the tibia against the femur , thus preventing tension on the surrounding tissue and additional soft tissue resection . however , care should be taken during insertion because the keel should be passed through a narrow small space in the flexed knee , and determining the exact insertion site can be challenging because the keel punch in the resected surface of the tibia is not easy to identify due to cement . therefore , we applied sufficient traction with the knee in flexed position during mini - keel plate insertion and used a custom made holder to securely hold and precisely insert the implant through a narrow space . although the length of the keel of the mini - keel tibial component is short compared to that of other tibial components , it provides 5.20% wider contact surface , thus not compromising stability of the knee . however , in an attempt to obtain further stability by increasing the interface between the implant and the tibia , we additionally used a 45 mm drop - down stem extension in all knees . benazzo and rossi6 ) first reported on the results of mis - tka using a modular tibial implant : the 5-year survival rate was 97.9% in 200 knees during the 3-year follow - up in the prospective study . bonutti et al.2 ) demonstrated favorable clinical and radiological results : the 9-year survival rate was 97.1% in 90 knees . in the current study , the survival rate was as high as 99.4% at 6.1 years after mis - tka using a mini - keel tibial implant , which can be attributed to appropriate realignment of the tibial component , proper selection for tibial fixation site , and the use of a drop - down stem extension for improved stability . seo et al.5 ) documented that surgical site healing was delayed and the incidence of infection was high after mis - tka that was performed under limited visualization of the operative field compared to that after conventional tka due to severe skin traction and prolonged surgical time . however , there was no case of infection except for 1 case that required revision due to infection at 6 years and 6 months after surgery . the infection rate in our patients was significantly low compared to that in previous studies , which we believe is attributable to careful application of traction to skin and the relatively short surgical time ( 87.512.4 min ) . most publications in the literature are reports on the short - term results of mis - tka using the conventional implants1,4,9,13,15,16,17 ) . thus , the significance of this study can be found in the fact that we investigated more than 5-year mid - term follow - up results of mis - tka using a tibial component designed specifically for mis in a relatively large study population . the limitations of this study include that the study design was retrospective and many of the 316 cases that underwent mis - tka were not included for analysis due to unavailability for outpatient clinic evaluation . in addition , all the operations were performed by the same surgeon experienced in mis - tka . although the procedure requires a long learning curve to achieve accuracy and reduce operation time1,8 ) , we think that long - term studies that involve multiple institutions and various surgeons should be conducted in the near future . the more than 5-year mid - term follow - up of mis - tka using a mini - keel modular tibial implant in 168 cases showed high implant survivorship and satisfactory clinical and radiological results .
purposeto evaluate the minimum 5-year mid - term clinical and radiological results of minimally invasive surgery total knee arthroplasty ( mis - tka ) using a mini - keel modular tibia component.materials and methodswe retrospectively evaluated 254 patients ( 361 cases ) who underwent mis - tka between 2005 and 2006 . 
 the latest clinical and radiological assessments were done in 168 cases that had been followed on an outpatient basis for more than 5 postoperative years . 
 clinical results were assessed using the hospital for special surgery ( hss ) score and knee society score . 
 radiological evaluation included measurements of knee alignment.resultsthe average postoperative knee range of motion and hss score were 134.312.4 and 92.77.0 , respectively . 
 the average postoperative femorotibial angle and tibial component alignment angle were 5.21.7 valgus and 90.21.6 , respectively . 
 the average tibial component posterior inclination was 4.82.1. the percentage of cases with tibial component alignment angle of 903 was 96.1% , and that with the femorotibial angle of 63 valgus was 94.0% . 
 radiolucent lines were observed in 20 cases ( 12.0% ) : around the femur , tibia , and patella in 14 cases , 10 cases , and 1 case , respectively . 
 however , they were less than 2 mm and non - progressive in all cases . 
 the survival rate was 99.4% and there was no implant - related revision.conclusionsmis-tka using a mini - keel modular tibial plate showed satisfactory results , a high survival rate , and excellent clinical and radiological results in the mid - term follow - up .