Knee flexion influences periprosthetic BMD measurement in the tibia. Suggestions for a reproducible clinical scan protocol.
OBJECTIVE
The quality and quantity of bone is important for the success of joint prostheses and may be monitored by dual energy X-ray absorptiometry (DXA). Available protocols suggest that the knee should be positioned in full extension. This is not possible for most patients in the first days after surgery; however, deficits in extension normalize with rehabilitation. Individual knee flexion between the baseline and follow-up investigations may therefore be different. We investigated the sensitivity of bone mineral density (BMD) measurements to knee flexion in a phantom study and in patients. We suggest a protocol for clinical use.


METHODS
2 phantom tibial bones with tibia components were secured in a clamp and BMD measurements were repeated 5 times at every 5 degrees change in flexion from 0 degrees to 20 degrees. For clinical use, a soft foam positioner was produced, in which the lower leg could be placed in neutral rotation and with the knee in approximately 25 degrees of flexion. The clinical repeatability was tested with double examinations in 38 patients. We investigated 3 regions of interest (ROIs) below the tibial plateau.


RESULTS
In the phantom study, just 5 degrees of flexion was found to change the measured mean BMD. The reproducibility of clinical measurements (coefficient of variation) in the 3 ROIs assessed ranged from 1.8% to 3.7% for the anteroposterior scans, and from 3.4% to 6.2% for the lateral scans.


CONCLUSIONS
Knee flexion does affect the measured periprosthetic tibial BMD, and knee flexion should be the same at all clinical follow-ups. The protocol and soft foam positioner that we suggest permit precise and reliable assessment of BMD in the proximal tibia and they can be used in clinical work.
Stilling
Maiken
M
Department of Orthopaedics, Aarhus University Hospital, Denmark. maiken.stilling@ki.au.dk
Søballe
Kjeld
K
Larsen
Kristian
K
Andersen
Niels Trolle
NT
Rahbek
Ole
O
D015502:Absorptiometry, Photon; D019645:Arthroplasty, Replacement, Knee; D015519:Bone Density; D006801:Humans; D007717:Knee / Q000502:physiology* / Q000503:physiopathology; D007720:Knee Prosthesis; D019047:Phantoms, Imaging; D011379:Prognosis; D016059:Range of Motion, Articular; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013977:Tibia / Q000000981:diagnostic imaging / Q000378:metabolism; D016896:Treatment Outcome
Spine (Phila Pa 1976). 2000 Feb 1;25(3):285
10703097
Calcif Tissue Int. 1998 Apr;62(4):359-61
9504962
J Bone Joint Surg Br. 2006 May;88(5):606-13
16645105
J Orthop Res. 2000 Jan;18(1):40-7
10716277
Clin Orthop Relat Res. 2008 Feb;466(2):380-8
18196421
Clin Physiol Funct Imaging. 2008 Mar;28(2):101-6
18076657
Clin Orthop Relat Res. 1995 Dec;(321):68-72
7497687
Int Orthop. 2008 Feb;32(1):47-51
17115154
Acta Orthop Scand. 2003 Dec;74(6):677-82
14763698
J Bone Joint Surg Br. 2003 Sep;85(7):975-9
14516030
J Clin Densitom. 2004 Fall;7(3):319-25
15319504
Lancet. 1986 Feb 8;1(8476):307-10
2868172
Arch Orthop Trauma Surg. 1999;119(5-6):324-6
10447632
J Arthroplasty. 2000 Sep;15(6):744-53
11021450
Acta Orthop Scand. 1999 Dec;70(6):603-8
10665727
Int Orthop. 2009 Dec;33(6):1495-500
19050883
Osteoporos Int. 2007 Nov;18(11):1515-23
17483864
J Bone Joint Surg Br. 1995 May;77(3):377-83
7744919
J Arthroplasty. 1995 Feb;10(1):7-11
7730833
Acta Orthop Scand. 1995 Dec;66(6):513-6
8553818
J Arthroplasty. 2001 Oct;16(7):893-900
11607906
J Bone Joint Surg Br. 1995 May;77(3):479-83
7744941
J Bone Joint Surg Am. 2004 Jul;86(7):1479-84
15252096
Clin Orthop Relat Res. 1989 Nov;(248):15-9; discussion 19-20
2805473
Acta Orthop. 2005 Aug;76(4):563-72
16195075
J Arthroplasty. 2007 Aug;22(5):689-91
17689776
Acta Orthop Scand. 2004 Oct;75(5):600-5
15513494
J Bone Joint Surg Br. 1994 Nov;76(6):912-7
7983118
J Arthroplasty. 1999 Dec;14(8):957-63
10614887
J Clin Densitom. 2004 Winter;7(4):424-31
15618604