Total knee arthroplasty (TKA) is performed to relieve pain or to restore limited function in patients with arthritis. The median parapatellar approach is the standard approach, and adequate knee exposure is crucial.3,35 However, the disadvantages of this approach include disturbing the blood flow to the patella and patellar tracking.7,19,26 Although more commonly reported in earlier studies, patellar complications after TKA remain a common cause of pain and reoperation.23,28 Patella-related complications such as subluxation, dislocation, patellar tilt, instability, and patellar fracture are the most common postoperative problems in TKAs.5,13 Perioperative disruption of the patellar blood supply may cause osteonecrosis, stress fracture, and loosening.6,20,31
Mini-incision TKA techniques were developed to achieve the results of a TKA, but with a smaller incision and reduced trauma to soft tissue and the extensor mechanism. The subvastus and midvastus approaches gained popularity because they preserve the extensor mechanism and patellar blood flow compared with the medial parapatellar approach.10,14 The distal portion of the arthrotomy in the midvastus approach is identical to the medial parapatellar approach until reaching the superomedial pole of the patella. At that point, the midvastus incision splits the vastus medialis obliquus (VMO) muscle in the direction of its fibers and extends medially toward the intermuscular septum.10 The VMO snip has been defined as a variation of the midvastus approach, but includes proximal and distal capsular release in addition to complete medial arthrotomy. The patella is subluxated (but not everted) during this modified midvastus approach.3 Incisions made proximal to the superomedial pole of the patella may damage the descending genicular artery.1,6
Blood supply to the knee is derived from an anastomosis of the five arteries, namely the superior medial and lateral, the middle (posterior), and the inferior medial and lateral genicular arteries.18 Patellar blood from the descending genicular artery is one of the most important arteries supplying the patellar rete. Surgical exposure during a TKA compromises blood flow to the patella. Devascularization of the patella has been reported to be an important technical factor in association with patellar fracture.1,4,6,27 The arteries contributing to the rete are crucial for vascularity of the patella and should be carefully preserved whenever possible.
Because of the potential vascular damage associated with a TKA, we sought to determine the course of the descending genicular artery before its entry to the patellar rete and its distance from the patellar margin of the incision, and to define a safety zone where the midvastus and VMO snip approaches could be safely performed.
MATERIALS AND METHODS
We dissected 30 knees from 15 formaldehyde-fixed human cadavers to determine the arterial blood supply to the patella and the distance of the descending genicular artery from the patellar margin of the midvastus approach. The mean age of the cadavers was 32 years (range, 21-43 years). Eleven specimens were male and four were female. Microscopic magnification (×12) was used for precise dissection. Dissections were performed by all of the authors (KB, BE, ET, AFE). The skin, subcutaneous fat, and superficial fascia were carefully removed. We then exposed the femoral artery after mobilizing the sartorius. Exposure was started proximally with blunt mobilization of the sartorius from the neurovascular bundle underneath as far as the distal edge of the vastoadductor membrane. The underlying vascular patterns were isolated after removing the vastoadductor membrane. The femoral artery of 30 pairs of cadaver legs were cannulated and infused with colored latex. The descending genicular artery and its branches were isolated from the point of origin and dissected to the patella (Fig 1).
We marked the entry points of the descending genicular artery into the parapatellar vascular network (the most prominent portion of the medial border of the patella), superomedial border of the patella, and the midpoint of the dorsal patellar surface. We documented various parameters of the descending genicular artery and its course to the parapatellar network of the knee (11 males, four females; bilaterally). The entry angle to the patellar rete was measured as the angle between the vertical axis of the patella and the course of the genicular artery. The location of the descending genicular artery near the patella was determined by the entry angle to the patellar rete at the superomedial pole of the patella. We also measured the distance of the artery from the superomedial pole of the patella and the most prominent part of the patella where it can be palpated easily during surgery. The patellar margin of the incision included splitting the VMO muscle belly in the direction of its fibers. The distance of the artery from the patellar edge of the midvastus and VMO snip approaches was measured from the superomedial pole of the patella and the more easily palpated medial pole of the patella.
All data were combined using descriptive statistics. Continuous variables were calculated as a mean and range.
The average entry angle was 33° (range, 20°-40°) (Fig 2).
The mean distances of the superomedial pole of the patella and medial tubercle of the patella from the descending genicular artery on the medial parapatellar arthrotomy line were 13.5 mm and 27.7 mm, respectively (Table 1). The descending genicular artery coursed as close as 6.9 mm from the superomedial pole where the arthrotomy angled medially toward the intermuscular septum.
A TKA requires extensile approaches with excellent access to the intraarticular and periarticular structures for proper implant positioning. The classic approach for a TKA is a medial parapatellar arthrotomy which is performed by an incision splitting the rectus femoris and vastus medialis proximally, and then curving inferiorly around the patella and just medial to the tibial tuberosity.16 Although a TKA is easily performed using this approach, it disrupts the medial patellar blood flow and extensor mechanism.1,6
We recognize the limitations of fixed cadavers that have limited tissue mobility. Although the anatomic data so derived might underestimate or overestimate the vascular injuries, arterial supply also might be vulnerable from stretch injury in more mobile tissues if the patellar mechanism is aggressively retracted. However these data provide information regarding the anatomic course of the descending genicular artery that might be helpful in its preservation.
Despite the high success rate of TKAs, postoperative complications remain a source of frustration for patients and surgeons. Patellofemoral complications are the most common and most frequent causes of reoperations.5,12,13,30 Although patellar fracture after a TKA is uncommon, with a reported prevalence of 0.05% to 21%, it is the most common periprosthetic fracture around knee.4,41 It has a multifactorial etiology, but seems to be associated with devascularization of the patella after a TKA, especially a lateral retinacular release with a medial parapatellar arthrotomy4,16,24,27,38 Weber et al reported 13 patella fractures in 1000 TKAs, nine of which were performed with a lateral release.39 Additionally, there were several case reports of avascular necrosis of the patella after TKAs in resurfaced and unresurfaced patellae.9,15,25,34
Mini-incision TKAs are gaining popularity because of an increased interest in preserving soft tissues to enhance postoperative recovery.3,29 Mini-incision TKAs are reported to reduce blood loss, hospital stay, and narcotic pain medication requirements, while increasing postoperative range of motion of the knee without compromising accuracy or short-term outcome.11,22 Splitting the vastus medialis muscle with patellar subluxation (rather than prolonged eversion), retraction of the patella (but not dislocation), and avoidance of gross dislocation of the tibiofemoral joint are suggested for a mini-incision approach.2,21,37 Numerous approaches have been advocated for mini-incision TKAs including the standard median parapatellar approach, subvastus approach, midvastus approach, and VMO snip.2,35 The main attraction of the midvastus approach has been its reported improvement in patellofemoral tracking with decreased incidence of lateral retinacular release and the possibility of preserving the descending genicular artery.8,40
The patella is supplied by patellar rete which is formed by the five genicular arteries.17 The descending genicular artery arises from the femoral artery before passing the adductor hiatus and gives off the saphenous, articular, and deep oblique branches.11 The retinaculum on the medial side is supplied by the anastomosis with the main contribution from the descending genicular artery.33 Preserving the descending genicular artery as one of the major contributors to patellar blood supply potentially may increase the quality of remaining bone and decrease patella-related complications (eg, avascular necrosis and patellar fracture). One of the two major medial arteries should be preserved to prevent patellar ischemia, which is observed after a medial parapatellar arthrotomy combined with a lateral retinacular release with excision of the fat pad.26
We studied the course of the descending genicular artery close to the midvastus and the VMO snip approaches. Cooper et al stated that the descending genicular artery is preserved in the midvastus approach.8 However, they provided no firm limitations for the patellar margin using this approach. A midvastus split can be performed at varying degrees from the superomedial corner of the patella. We found patellar eversion was easier with a horizontal incision; however, the risk of arterial damage increased. Our data show that the course of the descending genicular artery in the VMO enters the patellar rete with a 33° angle (range, 20°-40°), forming an arterial circle. This angle was similar to the vastus medialis insertion angle in most of the specimens, and the range highlights the importance of the incision direction to protect the descending genicular artery.32,36 The distance from the patellar margin of the incision was as close as 6.7 mm from the superomedial corner. The distance from the medial corner of the patella (where it can be palpated more easily during surgery) was as close as 18.4 mm. Both values indicate a high risk of arterial damage with the horizontal midvastus and VMO snip approaches. To define a safety zone where the midvastus and VMO snip approaches can be performed safely, we suggest a maximal safe distance for the medial arthrotomy of 15 mm from the superomedial pole of the patella and 30 mm from the most prominent medial corner in an adult within a 50° angle (Fig 2). We also measured the distance from the most superior portion of the patella because it is easily palpated during surgery. However, it may be difficult to locate the superomedial corner of the patella. Proximal capsular release and medial parapatellar arthrotomy can be performed safely within these limits. If surgical approaches to the knee are performed within these safety margins, dissection should stop short of the descending genicular artery, which may increase the potential to preserve the descending genicular artery in vastus-oriented approaches Although we suggest a safe area for protecting the descending genicular artery, the amount of increased blood supply and clinical importance of maintaining this artery have not been determined. Additional prospective studies are needed to establish whether sacrificing or preserving these vessels has any impact on the clinical outcome.
Mini-incision surgical approaches have been introduced to decrease soft tissue trauma, postoperative pain, and hospital stays and to increase rehabilitation. The reduced incision lengths and exposure necessitated new approaches such as the midvastus and VMO snip. The concept of minimal invasiveness includes decreased soft tissue disruption and a vastus medialis muscle split with patellar subluxation (but not dislocation) and avoidance of gross dislocation of the tibiofemoral joint. If the midvastus or VMO snip approaches are used, the incision should turn within 15 mm from the upper pole of patella and within 50º to preserve the descending genicular artery and its branches to the patellar rete.
We thank the staff of the Anatomy Department at the University of Ankara for help and advice with the dissections.
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