The effect of variaion in medial offset correction on functional outcome after total hip arthoplasty- a pilot study

Krishnan S.P*, Zaghloul A M*, Carrington R.W.Jж, Jeffery R. M.ж, Garlick N.ж

*Watford General Hospital, London, UK.
ж Royal Free Hospital, London, UK.  

1. All underwent primary cemented total hip replacements with no previous operations on their hip joint and had a normal contra-lateral hip.
2. Minimum post operative follow-up of 1 year at the time of evaluation.
3. No significant co-morbid conditions which might affect their functional outcome.
4. No post-operative infection or loosening.
5. The surgery was performed using the standard CPT femoral stem prosthesis (Zimmer UK) and using the Hardinge approach.

Patients were functionally assessed using the Harris hip scoring system⁶ and the delayed Trendelenburg test ⁷. Measurements were taken from standardised post-operative anteroposterior radiographs of the pelvis centred on the symphysis pubis, with the patient lying supine and the great toes touching to maintain consistent femoral rotation. The medial offsets on the operated side and the contra- lateral normal side were measured on the radiographs. Medial offset was measured as the perpendicular distance from the centre line of the proximal femur (mid – diaphyseal axis) to the centre of rotation of the femoral head (fig 1) ⁸. The centre of the femoral head was identified using templates of concentric circles.
The patients were divided into three different groups based on the study by Dolhein et al ⁸:

1. The normal offset group: The reconstructed offset was within +/-4mm of the offset measured on the contra-lateral normal hip⁹.

2. The increased offset group:The reconstructed offset was 4mm or more than the contra-lateral normal hip.

3. The decreased offset group The reconstructed offset less than 4mm compared to that of the contra-lateral normal hip.

All the patients had a diagnosis of osteoarthritis affecting only one hip with preservation of anatomy of the contra-lateral hip joint. All total hip replacements were performed by one experienced hip surgeon (N.G). Intra-observer and inter-observer variations were determined from ten randomly selected sets of radiographs measured initially and after three weeks by the same author and also by independent assessment of another author.Intra class correlation was then measured and was evaluated using the grouping recommended by J R Landis etal for the kappa statistics¹⁰. Scores between 0.61 and 0.8 represented substantial agreement and those greater than 0.81 almost perfect agreements.

Results :

Inter- and intra-observer variability showed almost perfect agreement while measuring femoral offset. (Table I)

Table I: reproducibility of measurements.

The mean age group of patients in this study was 71.1 years ranging from 45 to 87 years. The mean follow up at the time of evaluation was 38 months ranging from 12 to 78 months. Of the fifty patients studied, 26 patients (52%) were found to have had their hip offset accurately reconstructed (normal offset group); 14 patients (28%) had their hip offset increased (increased offset group) and 10 patients (20%) had their hip offset decreased (decreased offset group) (Table II). The variation in offset ranged from +20 mm to -15mm.

Table II: Summary of findings.

The mean Harris hip scores for patients with normal offset, increased offset and decreased offset were 92.01(range: 68.86-100), 90.25(range:72.8-99.8) and 91.50(range: 90.8-100) respectively. Harris hip scores were compared between the three groups using the Kruskal-Wallis analysis of variance, as it could not be assumed that the scores would follow a normal distribution (Fig 3). The p-value was 0.57, indicating no evidence of a difference in scores between the groups. Of the fifty patients studied the delayed Trendelenburg test could be performed in 43. Of these, 21 patients were from the normal offset group, 14 from the increased offset group and 8 from the decreased offset group. The number of patients who were found to be positive was 1, 0 and 1 in the normal offset reconstruction group, increased offset reconstruction group and the decreased offset reconstruction group respectively.

Box and whisker plot to compare the distribution of the Harris hip scores among the three offset groups. Thick horizontal lines represent median hip score, the box represents the inter quartile range and whiskers represent the full range.

Discussion:

Increased offset reconstruction has been shown to increase the range of motion of the reconstructed hip and the abductor muscle strength¹. Vaz et al found that although the hip abductor muscle strength is related to its function, isometric measurements of strength should not be relied upon as the sole predictor of function in patients who have undergone total hip arthoplasty ¹¹. Although an increased offset may increase the isometric abductor muscle strength as suggested by Mcgregory et al¹; our study did not show any significant difference in the functional scores among patients with higher offset reconstruction. Decreased offset shortens the moment arm of the abductor musculature and results in increased joint reaction force.  This may in turn lead to bony impingement, limp and increased polyethylene wear ¹². Decreased offset also causes reduction in the soft tissue tension whereas and an increase in offset may result in increased stress with in the stem and the proximal medial cement mantle ^13,8,14. Major overcorrection of the femoral offset may also lead to soft tissue problems such as pain resulting from trochanteric prominence and from bursitis ³. Whether the reconstructed offset should be equivalent to that of the contra-lateral normal hip (if the contra-lateral hip joint is available for preoperative planning) or whether it should be deliberately increased remains unclear. Harris pointed out that extreme ranges of motion at the hip might not determine the functional outcome as they are not important for normal daily activities. Pain and functional capacity are the two basic considerations in the functional assessment using the Harris hip scoring system⁶.They constitute the indications for surgery in the vast majority of patients with hip problems and hence receive the heaviest weighting on hip scoring. In specific cases, correction of deformity or restoration of motion may be of prime importance but such cases are uncommon. This study supports these ideas and shows that even though an increased offset may produce an additional increase in the replaced hip’s range of motion¹; such an increase does not determine the functional score of the patient as extreme ranges of motion are not important in normal daily activities. In conclusion, no significant difference was found in the mean hip scores and the delayed Trendelenburg tests between the three groups of patients in this pilot study regardless of the offset at the hip. A larger size study would be required to confirm these results.

References:

1. Mc Grory BJ, Morrey BF, Cahalan TD, Carbanela ME: Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg Br.1995 Nov; 77(6): 865-9.

2. Davey J R, O’Connor D O, Burke D W, Harris W H. Femoral component offset. Its effect on strain in bone-cement. J Arthroplasty.1993 Feb; 8(1): 23-6.

3. Isao Asayama, Masatoshi naito, Motoyuki Fujisawa, Taichi Kambe.Relationship between radiographic measurements of reconstructed hip joint position and the Trendelenburg sign. J Arthroplasty Vol.17 No.6 2002:747-51.

4. Bourne R B, Rorabeck C H. Soft tissue balancing: the hip.J Arthroplasty 2002 Jun;17(4 Suppl 1): 17-22.

5. Kleemann RU, Heller MO, Stoeckle U, Taylor WR, Duda GN. THA loading arising from increased femoral anteversion and offset may lead to critical cement stresses. J Orthop Res. 2003; 21(5):767-74.

6. Harris W.H: Traumatic arthritis of the hip after dislocation and acetabularfractures: Treatment by mould arthroplasty J Bone Joint Surg P.737 , 51-A,No.4,June 1969.

7. Hardcastle P,  Nade S. The significance of the Trendelenburgh test.J Bone Joint Surg Br1985; 67:741.

8. Dolhain P, Tsigaras T, Bourne R.B, Rorabeck C.H, Mac Donald S, Mc Calden R. The effectiveness of dual offset stems in restoring offset during total hip replacement. Acta Orthopaedica Belgica, Vol.68-5-2002: 490-99.

9. Krishnan SP, Carrington RW, Mohiyaddin S, Garlick N. Common misconceptions of normal hip joint relations on pelvic radiographs.J Arthroplasty. 2006 Apr;21(3):409-12.

10. Landis JR, Koch G. The measurement of observer agreement for categorical data.Biometrics 1977;33:159-74.

11. Vaz MD, Kramer JF, Rorabeck CH, Bourne RB: Isometric hip abductor strength following total hip replacement and its relationship to functional assessments. J Orthop Sports Phys Ther. 1993 oct; 18(4):526-31.

12. Sakalkale DP, Sharkey PF, Eng K, Hozack WJ, Rothman RH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop 2001 jul;(388):125-34.

13. Campbell’s operative orthopaedics. 10th edition.vol.1.P 320.

14. Chang P.B; Mann K.A; and Bartel D.L: Cemented femoral stem performance- effect of proximal bonding, geometry and neck length. Clin Orthop., 1998; 355:57-69.

This is a peer reviewed paper 

Please cite as : Krishnan S.P:The effect of variaion in medial offset correction on functional outcome after total hip arthoplasty- a pilot study

J.Orthopaedics 2007;4(1)e9

URL: http://www.jortho.org/2007/4/1/e9

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