Knee extension range of motion and self-report physical function in total knee arthroplasty: mediating effects of knee extensor strength.

0.58 MB·English

by  Pua, Yong-Hao

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RESEARCH ARTICLE Open Access
Knee extension range of motion and self-report
physical function in total knee arthroplasty:
mediating effects of knee extensor strength
Yong-Hao Pua1*†, Peck-Hoon Ong1†, Hwei-Chi Chong1, William Yeo2, Celia Tan3 and Ngai-Nung Lo4
Abstract
Background: Knee extensor strength and knee extension range of motion (ROM) are important predictors of
physical function in patients with a total knee arthroplasty (TKA).However, the relationship between the two knee
measures remains unclear. The purpose of this study was to examine whether changes in knee extensor strength
mediate the association between changes inknee extension ROM and self-report physical function.
Methods: Datafrom 441patients with a TKA were collected preoperatively and 6 months postoperatively.
Self-report measure of physical function was assessed by theShort Form 36 (SF-36) questionnaire. Knee extensor
strength was measuredby handhelddynamometryand knee extension ROM by goniometry. Abootstrapped cross
product ofcoefficients approach was used to evaluate mediation effects.
Results: Mediation analyses, adjusted for clinicodemographic measures,revealed that theassociation between
changes in knee extension ROM and SF-36 physical function was mediatedby changes in knee extensor strength.
Conclusions: In patients with TKA, knee extensor strength mediated the influence of knee extension ROM on
physical function. These results suggest thatinterventions to improvethe range of knee extension maybe useful in
improving knee extensor performance.
Background Although most [5-12] but not all [13] previous studies
Knee osteoarthritis (OA) is associated with substantial have implicated knee strength and knee extension ROM
functionallimitationsinolderadults[1].Specifically,knee as important predictors of physical function, notably
OA,alongwithhipOA,accountedforthelargestpropor- absent from these studies was an evaluation of the inter-
tion of disability in walking and stair climbing more than relation between knee extensor strength and knee exten-
any other chronic diseases [2]. In patients with advanced sion ROM: previous studies – by their choice of standard
stages of painful knee OA, although a total knee arthro- regressionanalyses–assumedthatthese2kneemeasures
plasty (TKA) can effectively restore function, 1 in 6 have distinct and unrelated etiologies and that they act
patients with TKA continue to have substantial physical throughdifferentpathwaystoinfluencephysicalfunction.
function limitations [3]. Clearly, an in-depth understand- Is it possible that knee extensor strength and knee
ing of the modifiable predictors of physical function and extension ROM are interrelated? Based on what is known
theiraetiologicalpathwaysisneededwhich,inturn,could aboutthelength-dependentnatureofmuscleforceproduc-
assisttherefinementofinterventionstoimprovefunction. tion, [14,15] we believe it is biologically plausible that knee
Clinically, two common knee impairments before and extensor strength may play a role in the mechanistic path-
following aTKA are reduced knee extensor strength and way connecting knee extension ROM and physical func-
deficits in knee extension range-of-motion (ROM) [4]. tion. Indeed, many activities of daily living – for example,
level walking [16] – require the knee extensors to produce
forcesatlesserdegrees(~30°)ofkneeflexion.Ostensibly,at
*Correspondence:[email protected]
†Equalcontributors a given knee joint angle near full extension, a flexion con-
1DepartmentofPhysiotherapy,SingaporeGeneralHospital,Singapore, tracture may limit force production because the knee
Singapore extensors – the monoarticular vastii in particular – are
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operating at a disadvantaged (lower) position on the outpatient physiotherapy clinic. The Centralized Institu-
ascending limb of their force-angle curves [14,15]. In as tional Review Board of Singhealth (CIRB), Singapore,
much as this concept may make sense and lead to the approvedthestudyandwaivedtheneedforinformedcon-
refinement of theoretical and intervention models, sup- sentduetotheretrospectiveandanonymousnatureofthe
porting data in patients with TKA are surprisingly study.
sparse.
Becausetheinterrelationbetweenkneeextensorstrength Demographicvariables
and knee extension ROM is possible and important to The demographic information collected in this study
understand clinically, we initiated the present study to includedage(inyears),sex,ethnicity(SingaporeanChinese,
examine whether changes in knee extensor strength me- as compared with others), and education (1=primary
diate the association between changes in knee extension school or less, 2=vocational or secondary school, 3=col-
ROM and self-report physical function in a large group of legeoruniversity).Dataaboutcomorbiditieswereretrieved
patientsbeforeandfollowingaTKA. frompatients’medicalrecordsbyaresearchassistantusing
achecklistmodeledaftertheSelf-AdministeredComorbid-
Methods ityQuestionnaire[17].Patients’heightandweightwerealso
Settingsandparticipants obtainedandbodymassindex(BMI)wascalculatedasthe
Our study used a longitudinal pre- and post-TKA design ratioofweighttosquaredheight(kg/m2).
thattookadvantageofthesubstantialdifferencesinchange
for knee impairments and self-report physical function. Selfreportphysicalfunction
The study involved 836 consecutive patients aged 50 years At each assessment, patients were interviewed in either
or older who underwent a primary TKA for knee OA English or Mandarin using the Short Form-36 [18], of
performed by three high-volume surgeons at Singapore which we used the physical function subscale as the
General Hospital, Singapore, from 3 January 2006 to 29 outcome variable and the bodily pain and mental health
January 2009. To avoid potential confounding effects from subscalesasthestudycovariates.Allsubscalesrangefrom
acontralateralTKAoperation,weexcludedpatientsifthey 0–100,withhigherscoresrepresentingbetterhealthstate.
had a contralateral TKA within the 12 months before or 6 The English and Chinese versions of the SF-36 have been
months after their indexTKA (n=152). We also excluded previouslyvalidatedforuseinSingapore[19].
patients who (i) had a history of stroke or other neuro-
logical disorders (n=30), (ii) had a history of lower limb Kneeextensorstrength
fracture (n=12), (iii) had previously undergone a hip A hand-held dynamometer (Lafayette, IN, USA) was
arthroplasty or high tibial osteotomy (n=26), (iv) had used to measure isometric knee extensor strength based
previously undergone a unicompartmental knee arthro- onthetesting procedures described byMartinet al.[20].
plasty (UKA) on their index knee (n=4), or (v) developed A total of five raters – two physiotherapists and three
medical or surgical complications prior to the follow up technicians – obtained the knee strength (and ROM,
session (n=64). Finally, because this exploratory study is vide infra) measurements. A “make” test was used as it
concernedwiththeeffectsofkneeflexioncontractures,we was deemed more reliable than a “break” test [21]. Knee
excluded patients with missing knee data (n=48) and extensor strength was measured with the patients in
patientswith knee hyperextension (extension ROM <0°) at supine position, and the knee was positioned in approxi-
the preoperative or follow-up assessment (n=59). (The mately 30° flexion by a firm wedge. The dynamometer
recruitment process is summarized in a flowchart in force padwasplaced justproximaltotheankle jointand
Additional file 1.) Thus, the sample for analysis comprised knee extensor strength was quantified in kilogram force.
the remaining 441 patients who underwent a preoperative All patients performed two maximal trials for 3 to 5
evaluationwithin5weekspriortotheiroperation.Ofnote, seconds with a 30-second rest interval. Additional mea-
this sample size was not based on a formal power calcula- surements were taken if the patient reported a failure to
tion but on all eligible patients in our database. Follow-up achieve maximum effort. The higher of two valid trials
assessment was conducted approximately 6 months after was recorded and normalized as a direct percentage of
theoperation.Alldatawerecollected,aspartoftheclinical body weight (%BW) because we believe that this index
process, by physical therapists and entered into an elec- of knee strength made intuitive sense to our patients.
tronic database per routine practice policies of our institu- Although we [22] and others [23] have shown, using
tion. All patients in this study were managed using a allometric analyses, that the association between muscle
coordinated clinical pathway to ensure standardized med- force generation andbodyweightis not a strict1:1 ratio,
ical, pharmacological, and rehabilitation care. Within two analyzing our data using allometrically-scaled knee
weeks post TKA, these patients began a 4- to 6-week re- extensor strength did not qualitatively alter our study
habilitation program at the Singapore General Hospital conclusions (data not shown). Also of interest, we
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performed strength testing in the supine position – and and percentages for categorical variables. Analysis of
not in the seated position – with the intent to reduce variance (ANOVA) for repeated measures was used to
the influence of the rater’s strength on the patient’s mea- test for change over time in the SF-36 and knee mea-
surements [20]. We attempted to ensure that the same sures. Pearson correlation was used to quantify the cor-
rater evaluated the same patient at each time point al- relation between kneemeasures.
though this was not fully achievable due to the large We used a mediation model to examine whether
patient volume in our hospital (~1,500 TKAs per annum knee extensor strength mediated the influence of knee
[24]). Nevertheless, all technicians were trained and extension ROM on SF-36 physical function at the
audited by authors HCC and WY who have a combined change scores (pre- to post TKA) level (Figure 1).
experience of over 30 years in orthopaedic and sports Covariates in each model included age, sex, BMI, the
physiotherapy. Furthermore, for the supine knee strength number of comorbidities, and changes in SF-36 bodily
measurements, previous studies have demonstrated good pain and mental health scores. In the mediation
test–retest reliability (intraclass correlation coefficients, model, the total effect (path c) of the independent
0.80to0.86)andconcurrentvaliditywithBiodexdynamo- variable (changes in knee extension ROM) on the
metrymeasurements[20,25]. dependent variable (changes in SF-36 physical func-
tion) comprises a direct effect (path c’) of the inde-
KneeextensionROM pendent variable on the dependent variable and an
A large standard goniometer (Jamar, Clifton, NJ, indirect (mediation) effect of the independent variable
USA) was used to measure passive knee extension on the dependent variable through the mediator
ROM. Knee extension ROM was measured with the (changes in knee extensor strength). The mediation
patients in supine position with the heel elevated on effect through the mediator is quantified by the pro-
a firm wedge. The axis of the goniometer was placed duct of the regression coefficient of knee extension
on the femoral lateral epicondyle. The proximal arm ROM on knee extensor strength (path a) and the re-
of the goniometer was directed toward the greater gression coefficient of knee extensor strength on SF-
trochanter of the femur whilst the distal arm of the 36 physical function (path b). We used the INDIR-
goniometer was directed toward the lateral malleolus ECT macro [27] to estimate the mediation effect
of the ankle. Patients were asked to relax to allow the (path axb) and its bootstrapped (1,000 samples), bias-
knee to passively extend. Two sets of measurements corrected and accelerated 95% confidence interval
were taken, and the lower measurement (greater knee (95% CI). Mediation effects are considered statistically
extension) was recorded. For the knee extension significant when zero is not contained within the 95%
ROM measurements, one previous study in patients CIs [27]. Finally, in sensitivity analysis, we tested a
with knee OA has demonstrated good test–retest reli- reversed mediation model – wherein changes in knee
ability (intraclass correlation coefficient=0.85) [26]. extension ROM were due to changes in knee extensor
strength – to assess the direction of the proposed
Statisticalanalysis mediation effects. All statistical analyses were done
We used descriptive statistics to characterize the study with PASW software, version 18, and R software, ver-
sample: we used means (SDs) for continuous variables sion 2.15.0.
Figure1Changes(pretopostTKA)inkneeextensorstrengthmediatedtheassociationbetweenchangesinkneeextensionROM
andchangesinSF-36physicalfunction.Forpresentationclarity,covariates–namely,age,sex,bodymassindex,numberofcomorbidities,
andchangesinSF-36bodilypainandmentalhealthscores–arenotdisplayed.
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Results Table2PostoperativerecoveryofSF-36andknee
Samplecharacteristics measures(n=441)
Table 1 summarizes the participants’ descriptive charac- Variables PreTKA PostTKA P-value†
teristics, while Table 2 provides the descriptive statistics SF-36variables
for the SF-36 physical function and knee measures. The
Physicalfunction 38.8(22.7) 66.6(17.9) <0.001
patients were predominantly female (82%) and were on
BodilyPain 36.8(18.3) 69.0(23.5) <0.001
average moderately overweight (mean (SD) BMI, 28(4.8)
kg/m2). All SF-36 and knee measures increased signifi- MentalHealth 75.9(19.1) 82.7(14.5) <0.001
cantly 6 months following aTKA. Specifically, mean SF- Kneevariables
36 physical function scores were 39(SD=23) points Extensorstrength,%BW 18.0(8.0) 25.6(8.6) <0.001
preoperatively and 67(SD=18) points 6 months post- ExtensionROM,deg 8.2(8.2) 4.3(4.9) <0.001
operatively. With respect to the knee measures, knee
Note:Valuesreportedweremean(SD)unlessstatedotherwise.
extensor strength increased from preoperative level by Abbreviations:SF-36=MedicalOutcomesStudy36-ItemShort-FormHealth
around40%atthe6monthpostoperativeassessment(18% Survey;ROM,rangeofmotion;BW,bodyweight;TKA,totalkneearthroplasty.
†P-valuesderivedfromrepeated-measureANOVA.
BWvs.26%BW;P<0.001).KneeextensionROMimproved
significantly(P<0.001)from8.2°(kneeflexioncontracture) extension ROM was significantly associated with increa-
preoperativelyto4.3°postoperatively.Improvementinknee singphysicalfunction(P<0.01forpathc, Figure1).Con-
extension ROM was positively associated with increased trolling for covariates, the mediation effect of knee
knee extensor strength (r=0.22, P<0.001, Figure 2). extensor strength on the association between changes in
Increased SF-36 physical function was positively associated knee extension ROM and physical function was statisti-
with improvement in knee extension ROM (r=0.15, cally significant (path axb=0.13; 95% CI, 0.05 to 0.23). In
P<0.001) and with increased knee strength (r=0.27, contrast,nomediationeffectwasobservedinthereversed
P<0.001)(scattergramsareshowninAdditionalfile1). mediationmodel(pathaxb=0.04;95%CI,-0.01to0.11).
Mediationanalyses Discussion
Figure 1 shows the results of the mediational analyses. The purpose of this study was to explore, in patients be-
Overall, the mediator model accounted for ~29% of the foreand 6-months followingaTKA, whether knee exten-
variance in physical function (P<0.001). Improving knee sion ROM had an indirect effect on SF-36 physical
function through the putative pathway of knee extensor
strength. Our results suggest that changes (pre to post
Table1Demographicsandpatientcharacteristics
(n=441) TKA) in knee extensor strength mediated the association
between changes in knee extension ROM and self-report
Characteristic Value
physical function. To our knowledge, these findings have
Age,years 67.9±7.8
notbeenpreviouslydescribedinpatientswithTKA.
Malesex,no.(%) 83(19%) Our results, without considering mediating effects, are
SingaporeanChinese,no.(%) 389(88%) consistent with cross-sectional studies [7,9,12] of apositive
Height,m 1.54±0.08 association between knee extension ROM and physical
function in TKA (path c in Figure 1). Furthermore, case
Bodymass,kg 65.9±13.1
series [28,29] and small trials [30] are available demonstra-
BMI,kg/m2 27.9±4.8
tingthatinpatientsbeforeandfollowingaTKA,rehabilita-
Overweightorobese,no.(%) 375(85%) tion interventions – for example, such as manual therapy
Education,no.(%) and splinting – increased knee ROM and improved phys-
Primaryorless 335(76%) ical function. Overall, our large study using longitudinal
Secondary 84(19%) change scores extends the previous literature to suggest
that knee extension ROM is an important correlate of
Tertiary 22(5%)
physicalfunctioninTKA.
Comorbidities,no.(%)
Consistent with theoretical expectations, changes in
None 66(15%)
knee extension ROM were associated with changes in
1comorbidity 141(32%) knee extensor strength (Figure 2) which, in turn, were
2comorbidities 137(31%) associated with changes in physical function (Figure 1).
3comorbidities 69(16%) Furthermore,theresultsfromthesensitivityanalysisusing
≥4comorbidities 28(6%) a reversed mediation model support the direction of the
proposed mediation. How do we explain our results? As
Note:Valuesreportedweremean±SDunlessstatedotherwise.
Abbreviations:BMI,bodymassindex. mentionedinthe Introduction,aknee flexion contracture
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r = 0.22 (0.13 to 0.31)
P < 0.0001
Figure2ScattergramofchangesinkneeextensorstrengthversuschangesinkneeextensionROMin441patients.Thestraightline
representsordinaryleastproductregressionlinewithits95%confidencelimits(curvedlines):(changesinkneeextensorstrength)=3.0(2.5to3.4)+1.18
(1.10to1.29)X(changesinkneeextensionROM).ObservedchangescoresofthekneeROMandstrengthmeasuresareindicatedbytheshortvertical
linesabovethex-andy-axes,respectively.
(knee extension ROM>0°) may potentially compress the improvingkneeextensionROMpost6monthsonkneeex-
force-length relationship [14,15] of the knee extensors tensor strength and functional ability. Finally, whilst we
suchthatstrengthproductiondiminishesatlesserdegrees used a self-report measure of physical function to facilitate
(~30°) of knee flexion. As a corollary, our results indicate data collection, we acknowledge that performance-based
that deficits in knee extension ROM are a putative multi- measures of physical function are necessary because they
variate cause of knee extensor weakness in TKA. And if provideimportant,complementaryinformationaboutfunc-
future studies can substantiate this notion, the discussion tionalstatus[35].
intheliteratureaboutwhetherkneeROMorkneemuscle
strength should be given greater prominence in TKA re- Conclusions
habilitation[31-33]wouldbecomemoot. Our results indicate that knee extension ROM is an im-
Our study has limitations. First, although our use of portant correlate of physical function in patients with
change scores permitted a more rigorous exploration of TKA. More important, changes in knee extension ROM
mediation than do cross-sectional data, our study variables and knee extensor strength interrelated to influence phy-
weremeasuredconcurrentlyateachtimepointwhichpre- sicalfunction.Iffuturestudiescoulddemonstratethatthe
cluded an examination of temporal associations between etiology of knee extensor muscle weakness is associated,
variables [34]. Accordingly, to better assess the criterion of inter alia, with knee extension ROM deficits, these fin-
temporality, future longitudinal studies should evaluate the dings would suggest that interventions to improve the
changes in the independent variable, mediator, and out- range of knee extension may be useful in improving knee
comes sequentially over different time points. Second, and extensorperformance.
relatedly,becausewedidnothavemultiplesequentialmea-
surements at successive time intervals, as pointed out by
Additional file
the reviewer, we were unable to evaluate whether the rate
of change in knee extension ROM influenced (moderated)
Additionalfile1:FigureS1. Flowchartofparticipantrecruitment.
theproposedmediationeffects.Third,wedidnothaveany TKA=TotalKneeArthroplasty,HTO=HighTibialOsteotomy,
follow-upstrengthmeasurementslaterthan6monthspost UKA=UnicompartmentalKneeArthroplasty.FigureS2.Scattergramof
changesinSF-36physicalfunctionscoresversuschangesinknee
TKA; hence, we were unable to evaluate the effects of
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