Knee Buckling: Prevalence, Risk Factors, and Associated Limitations in Function

Context Knee buckling is the sudden loss of postural support across the knee at a time of weight bearing. Its prevalence and consequences are not clear. Contribution This study of 2351 community-dwelling, middle-age and older adults found that 278 participants (12%) reported at least 1 episode of knee buckling in the past 3 months. Of these, 13% fell during the episode. Knee pain, quadriceps weakness, and worse physical function were associated with buckling. Caution The study's cross-sectional design limits causal inferences. Implications Knee buckling occurs commonly among middle-age and older adults and is sometimes associated with functional limitations. The Editors Knee buckling is the sudden loss of postural support across the knee at a time of weight bearing. Affected persons often characterize this phenomenon as giving way. One study has suggested that the prevalence of knee buckling is high in selected persons seeking physical therapy and stability training for knee osteoarthritis (1). However, the prevalence of knee buckling in the community and its effect on physical function have not been described. Buckling occurs mostly in persons with knee pain, and frequent knee pain affects about 25% of adults (2). Many of these persons have osteoarthritis of the knee (3). Whereas buckling and instability are a focus of orthopedic literature, these phenomena are neglected in medicine textbooks in chapters on knee pain or osteoarthritis (4, 5). When buckling is discussed, it is identified as evidence of an internal derangement, such as an anterior cruciate ligament (ACL) tear (5). A search of MEDLINE for articles on knee instability (subject), buckling, or giving way (words in title or abstract) from 1966 through June 2007 revealed that articles on knee buckling or instability were found almost exclusively in the orthopedic literature, where it was noted as a complication of surgery (6, 7); a hallmark symptom of ACL tear (8); or a consequence of specific, uncommon conditions, such as patellar dislocation (9). Thus, buckling is not generally described in native, uninjured knees. Buckling and symptoms of impending falling may be treatable or at least prevented, but avoiding activities that precipitate buckling may limit function. Buckling may cause falls and fractures and may help to explain the increased risk for hip fracture in patients with osteoarthritis who have higher bone density than others their age and who, therefore, should be at diminished risk for fracture (10). We sought to characterize the frequency of knee buckling in the previous 3 months among persons from the community. We also evaluated whether buckling was associated with particular characteristics, such as knee or other joint pain or muscular weakness. Finally, we examined the relationship of buckling with physical function and determined whether, independent of knee pain, buckling was associated with limited function. Methods Participants Our study cohort consisted of members of the Framingham Offspring Study and a newly recruited cohort from Framingham, Massachusetts. We combined these participants into a single cohort that we designated the Framingham Osteoarthritis Study cohort. Participants were examined between 2002 to 2005. Participants in the Framingham Offspring Study included surviving descendants and spouses of descendants of participants in the original Framingham Heart Study. The Framingham Osteoarthritis Study is a population-based study of osteoarthritis. As part of a study of the inheritance of osteoarthritis, descendants of the original Framingham Heart Study cohort (the descendants of the original cohort and their spouses constitute the Framingham Offspring) whom we had studied for knee or hand osteoarthritis in earlier Framingham Osteoarthritis studies (11) were selected. This allowed us to examine inheritance patterns of osteoarthritis and genetic linkage. Selected Framingham Offspring were originally examined from 1992 to 1995 (11). All surviving members of this group and those not lost to follow-up were contacted by a letter of invitation, and those interested in participating received a follow-up telephone call to schedule clinic examinations (Appendix Figure 1). Appendix Figure 1. Study flow diagram: the Framingham Offspring Study cohort. ACL = anterior cruciate ligament; MRI = magnetic resonance imaging. The newly recruited participants to the Framingham Osteoarthritis Study were drawn from a random sample of the Framingham, Massachusetts, community. Participants were recruited by using random-digit dialing and U.S. census tract data from 2000 to ensure inclusion of a representative sample of the community (Appendix Figure 2). To increase participation of eligible persons in contacted households, a press release was sent to the local media and public officials and flyers were hung in public areas to heighten awareness of the study, which focused on musculoskeletal health. To be included, persons had to be at least 50 years of age and ambulatory. Bilateral total knee replacement and rheumatoid arthritis were the exclusion criteria. Rheumatoid arthritis was assessed by using a validated survey instrument (12) supplemented by questions about medication use that would reflect treated disease. Participant selection was not based on the presence or absence of knee osteoarthritis or knee pain. Appendix Figure 2. Study flow diagram: the Framingham Osteoarthritis Study community cohort. ACL = anterior cruciate ligament. *Members of Framingham Offspring Study, positive screening for rheumatoid arthritis, magnetic resonance imaging (MRI) contraindicated, bilateral knee replacement, dementia or terminal cancer, or planned to move from area. Declined to participate because of cancer, chronic illness, no interest when received full details of the study, no reason given, no time, declined MRI or radiography, or other reasons. Not done because of claustrophobia, medical contraindications, or problems with scheduling. The study was approved by the Boston University Medical Center institutional review board. All participants provided written informed consent. Assessment of Buckling We informed all participants that we are interested in knee buckling, which is also called giving way. We asked, Have you had an episode in the past 3 months where your knee buckled or gave way? Persons who answered yes were asked to indicate which knee gave way, how many times in the past 3 months they had had such an episode, and whether knee buckling precipitated a fall. We also asked what they were doing when their knee buckled and offered 4 options (they could choose more than 1): walking, going up or down stairs, twisting or turning, or other. We chose a 3-month period because other studies have suggested that recollection of falling was accurate for approximately 3 months after the event (13). We considered a person who answered yes to the initial question on buckling as having experienced buckling. We also examined the subgroup of participants who had had more than 1 episode of buckling in the past 3 months. Pain, Physical Limitation, and Assessment of Risk Factors We asked participants about knee symptoms by using the following question: In the past 30 days, have you had any pain, aching, or stiffness in either of your knees? We considered all persons who said yes to have knee pain. A positive response triggered the follow-up question, Is the pain, aching, or stiffness in your right knee, left knee, or both knees? We assessed knee pain in the past week by using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire, a validated instrument for assessment of knee pain and disability (14). To evaluate the effect of buckling on physical function or limitation, we used WOMAC and the Short Form (SF-12) as self-reported measures of physical function or limitation. The WOMAC has a physical function subscale consisting of 17 questions, each of which asks about a different type of activity and whether knee problems limit the respondent in performing those activities. Each item is scored on a scale of 0 to 4 on the basis of the amount of limitation experienced; the total score ranges from 0 to 68, with 68 constituting profound limitation and 0 constituting none. In addition, we used items from the SF-12 (15, 16) to gather information on specific physical functional limitations that might be affected by buckling. The items we focused on from the SF-12 were whether participants were limited in moderate activities, in climbing several flights of stairs, and in the type of work or other activities they could do and whether they accomplished less than they wanted. Isometric quadriceps strength was measured while participants were sitting in a straight-backed chair by using a strain gauge dynamometer strapped to the lower leg. The force exerted when the knee was extended was recorded. Three measurements were made on each leg, and the maximum of the 3 was chosen as the measure of leg strength. For a person, we used the maximal leg strength. More than 90% of participants had all assessments completed during 1 clinic visit. Occasionally, participants were scheduled to return within 2 weeks for magnetic resonance imaging (MRI). Radiographic Assessments All participants underwent bilateral weight-bearing radiography using a posteroanterior fixed-flexion approach with a SynaFlex frame (Synarc, San Francisco, California), and a lateral weight-bearing semiflexed film was obtained according to a recently published protocol (17). Radiographs were scored on the KellgrenLawrence scale (18); a knee was considered to have radiographic osteoarthritis if its grade was 2 or greater. Patellofemoral osteoarthritis was characterized on the lateral view by using a validated approach (19). A bone and joint radiologist and an experienced rheumatologist each read roughly one half of the films. The intrareader value for KellgrenLawrence grade was 0.82, an