This study evaluated knee strength and mechanics during walking in patients with either a BKR or TKR in comparison to healthy control subjects. During strength testing, both TKR and BKR patients were able to exert maximal contractions with their knee extensors and flexors. However, TKR extensor strength was 34% less than the control knees. The BKR extensor strength was not significantly different than the control group and was 20% greater than the TKR group. The significant improvement of the BKR extensor strength may result from the individualized nature of the BKR design and procedure, which removes less bone and soft tissue. In a little over a year, the BKR patients were able to elevate their extensor strength to a level similar to the control population. Unfortunately, both BKR and TKR limbs showed weaker knee flexors than the control with a strength deficit of 32% and 28%, respectively, possibly due to both groups having an average BMI >30. None of the patients regularly participated in fitness or strength training programs focusing on the knee after the standard rehabilitation. Strength may be improved for both implant groups if patients participate in regular fitness programs and strength training within one year of surgery. Nonetheless, both the BKR and TKR patient groups reported good KOS-ADL scores at the time of testing. The higher KOS-ADL scores in BKR patients may be a result of the significant improvement of the BKR extensor strength when compared to the TKR patients.
During walking, TKR patients exhibited weakness in the sagittal plane. On average, the TKR group generated 40% and 24% less extensor moment than the control and BKR groups, respectively. The significant inability of the TKR knees to produce adequate extensor moment during walking is associated with the knee strength deficit. Due to the decreased extensor moment, TKR patients experienced increased duration of double limb support that was 36% and 41% longer than the control and BKR patients, respectively, during stance phase of gait. With smaller knee extensor moment and longer double-support time, TKR patients demonstrated a 20% and a 14% slower walking pace than the control and BKR patients, respectively. Although it is not uncommon to see knee replacement patients walking at a reduced speed with decreased knee extensor moment after one year of surgery, the BKR patients were able to improve their knee extensor moment and walking speed to a level similar to the controls. The advantages associated with the BKR, such as the individualized nature of the implant, small bone cuts, and reduced soft tissue intervention, may have contributed to these significant improvements.
Both the BKR and TKR knees were found to be well aligned. During stance of walking, the peak abduction angles of the BKR and TKR knees were as small as that of the control knees. It is also encouraging to see that during the loading response phase of walking, the peak abduction moment of both implant groups were close to that of the controls. With small peak abduction angle and peak abduction moment, the artificial knees in this study are expected to experience reduced mechanical stress in the medial side of the knees during daily walking, which could lead to increased implant survivorship.
During push-off, TKR knees exhibited a significant reduction of the peak abduction moment when compared to the BKR and controls limbs. This pattern may be related to the strength deficit of the TKR knees. It is possible that the reduced extensor moment during stance led to a reduction in medial knee loading signified by the decreased knee abduction moment. However, both the BKR and control knees produced similar amount of abduction moment at push-off, which corresponded to the similar extensor moment between the two knees.
It has been well documented that normal knees exhibit internal rotation of the tibia during flexion [12, 13]. We observed that the control knees displayed normal tibial rotation, which was represented by a 7° internal rotation during the stance phase of walking. When kinematics of the knee replacements were examined, the BKR knees produced a comparable amount of internal rotation (6°) to that of the controls. The TKR knees also displayed some internal rotation (3°), but the range of the rotation was limited and was 50% less than that of the BKR and control limbs. Ideally, knee replacement systems should help patients obtain natural knee kinematics during weight-bearing activities. Natural knee kinematics could promote a natural feel for patients while going through rehabilitation programs and may accelerate the recovery of knee strength and function. BKR patients demonstrated comparable knee kinematics as assessed from knee rotation to the controls as well as satisfactory knee strength at one year post-surgery. A recent study showed that customized TKRs manufactured using the same CT image to implant process are better suited to replicate normal knee kinematics when compared to non-customized TKR . Thus, the individualized shape of the customized implant may play an important role in improving kinematic function.
As with any study, there are some limitations that need consideration when interpreting our results. The sample size of participants in each group was smaller than the typical follow-up studies that report on functional and clinical end-points. Though sample size plays an important role in interpreting results, we believe from our prior experience with conducting such studies [9, 15,16,17,18], that the sample size chosen was adequate to enable us to make conclusions on our analyses. Additionally, we were able to maintain a similar sample size in each arm of the study. This should alleviate any bias due to sample size in any one study arm. Although participants in the control group were younger with smaller BMI than the other groups, the two patient groups were age, mass, and height matched. We believe that any advantage drawn from this would affect the implant groups equally, thus making comparisons between the implant groups relevant, while still providing context on how they compare to healthy controls. Ideally, we would have liked to test patients pre- and post-operatively and compare results with the patient being their own control. However, this would mean having to test patients that have end-stage OA, which we felt would not provide a clear comparison to healthy controls. Lastly, in this study, patients’ pre-operative Knee Society scores and gait analysis data were not available due to our cross-sectional study design. However, we believe our patients’ pre-surgical conditions were similar to patients used in other prospective studies examining functional improvements after knee replacements. In those studies, patients’ combined Knee Society scores were close to 100 and knee range of motion was around 120° [19,20,21,22]. In general, patients with end-stage knee OA experience joint pain and stiffness, which leads to functional limitations of performing daily activities such as walking, going up and down stairs, and rising from a sitting position. We chose the KOS-ADL because it is an effective instrument for measuring functional limitations associated with pathological disorders of the knee . However, we only administered the KOS-ADL during patients’ post-operative laboratory visit. Ideally, if the KOS-ADL score was obtained prior to surgery, then it is possible to quantify how much functional improvement is made at the time of the post-operative laboratory testing.
In summary, after one year post-surgery, both patient groups had good KOS-ADL scores with the BKR patients reporting higher satisfaction while performing activities of daily living. Patients with BKR knees were able to improve their quadriceps strength to a level that is close to healthy controls. BKR patients could walk at a speed similar to the controls with adequate extensor moment. BKR knees also revealed good knee mechanics represented by a small abduction angle, normal abduction moment, and normal internal rotation, which were comparable to those of healthy controls. The TKR knees had deficits in strength that led to a reduced walking speed and decreased extensor moment. Some improvements in knee mechanics were shown by the TKR limbs. Similar to the other groups, the TKR knees displayed a small abduction and normal abduction moment. Improved results seen in the BKR patients can be attributed to the ability of a BKR surgery to maintain both cruciate ligaments as well as the customized nature of the implant facilitating a more normal kinematic pattern. As the aging population increases in the US, it is expected that approximately 3.48 million patients will receive TKRs in 2030 . Studies have shown that nearly 30% of TKR recipients exhibit OA limited in the medial/lateral compartment and the patellofemoral joint . These patients benefit from BKRs, as this study shows that BKR patients exhibit better strength and mechanics while performing activities of daily living. Patients with modern TKR systems such as the one examined in this study may consider participating in strength training programs for further improvement in knee strength and function.