Patellar height may be important in determining function after total knee arthroplasty (TKA). By altering patellofemoral joint mechanics, patella baja may cause several functional issues after TKA.1-8 Patella baja leads to decreased range of motion (ROM) affecting both extension and flexion.5,8,9 Deep flexion can be restricted in TKA patients with patella baja because of tracking limitations associated with an inferiorly displaced patella. As the knee is brought into flexion, the patella can impinge on the anterior aspect of the tibial polyethylene or the tibial tray—presenting a true block to flexion and potentially altering wear.1,10
Another functional issue with patella baja is loss of strength in the extensor mechanism. The patella serves as a fulcrum for the extensor muscles of the knee. When positioned properly and functioning properly, the patella increases the extensor forces generated. When the patella is positioned in baja, the knee generates decreased extensor mechanism force.6,7 This can result in a lag, with the patient being unable to fully extend the knee. Extension-dependent activities are impaired. Patients with weak extensor function can experience poor function with stair climbing, rising from a chair, and exiting an automobile. The improper function and scarring of the patella can result in increased anterior knee pain and worse functional outcome scores after TKAs.3,9
An abnormally positioned patella can either result from or lead to increased scarring in the knee.9,11 Patellar height is often measured with the Insall-Salvati ratio (ISR), which is the patella tendon length (measurement of the tendon from the tibial tubercle to the inferior pole of the patella) divided by the patellar length (longest measured dimension of the patella) (Figure 1).12 Patella baja is defined as an ISR of less than 0.8. Other indices that reference off the tibial plateau (Blackburne-Peel ratio, Canton-Deschamps ratio) reflect an elevation of the joint line, or pseudobaja, and are unreliable for analysis of patella baja after TKA.13
Postoperative patella baja has been reported in 10% to 34% of primary TKAs.4,7 Inferior positioning of the patella and scarring can cause intraoperative difficulty with exposure and may complicate outcomes.9,13 The exposure scar is often larger in TKA revisions for infection compared with primary TKAs.
We conducted a study to compare the incidence of patella baja in noninfected and infected TKA revisions. We hypothesized that, compared with noninfected knees, infected knees treated with nonarticulating spacers would have a higher incidence of patella baja both before and after surgery secondary to more inflammation, immobilization, and related scarring.
Materials and Methods
We conducted a retrospective case–cohort study of 148 consecutive TKA revisions. All TKA revisions were performed between 2003 and 2009 using a mobile-bearing revision system from a single manufacturer. All surgeries were done at a single institution by the 2 senior surgeons. The surgical approach was a standard medial parapatellar approach without patellar eversion. Our institutional review board approved the study and waived the requirement for informed consent, as this was a retrospective study of existing medical records that posed no more than minimal risk to patients.
To properly evaluate patellar height, orthopedic specialty–trained radiologic technicians obtained preoperative and postoperative weight-bearing radiographs using a standardized lateral radiograph in clinic. Two blinded investigators measured ISR radiographically both before surgery (preexplant for septic revisions) and at latest follow-up (postreplant for septic revisions). Patients with inadequate films and/or patellectomies were excluded, along with patients who had less than 6 months of postoperative follow-up.
Ninety-one patients (101 TKAs) met the study inclusion criteria. Two groups of cases were compared: aseptic revisions (n = 67) and septic revisions (n = 34). Reasons for aseptic revisions included implant loosening (24/67, 35.8%), instability (12/67, 17.9%), pain (12/67, 17.9%), lysis (5/67, 7.5%), stiffness (3/67, 4.5%), and malrotation (2/67, 3.0%). Infection was determined by Musculoskeletal Infection Society criteria, as documented by positive aspirations and/or intraoperative tissue cultures taken at prosthesis explantation, elevated white blood cell count in the aspirate, elevated percentage of polymorphonuclear (PMN) cells in the aspirate, gross purulence, presence of chronic draining sinus, or histologic analysis revealing acute inflammation with more than 5 PMN cells per high power field.14,15
All infected TKAs were treated with 2-stage revisions. The standard of care at our institution through this series was to use a nonarticulating spacer for the treatment of infection. Weight-bearing status varied by extent of bone damage. Six weeks of culture-specific intravenous antibiotics were administered with assistance from an infectious disease consultant. Reimplantation was performed when clinical and laboratory criteria for resolution of infection were met—specifically, when erythrocyte sedimentation rate was less than 30 mm/h, C-reactive protein level was less than 10 mg/L, and aspirates were culture-negative. Mean (range) follow-up was 33.9 (6.2-75.7) months for aseptic revisions and 32.3 (7.5-94.2) months for septic revisions. Radiographic follow-up was performed at each visit, with weight-bearing anteroposterior and posteroanterior views, along with a lateral knee radiograph. At final follow-up, ROM was recorded by the senior attending evaluating the patient.