Strength and Voluntary Activation of Quadriceps Femoris Muscle in Total Knee Arthroplasty with Midvastus and Subvastus Approaches

Material and Methods 

Twitch-Interpolation Technique 

If voluntary muscle fibers are completely activated, superimposed external muscle stimulation (twitch) will not produce any additional muscle force. Conversely, if voluntary muscle contraction is incomplete, additional force is generated by superimposed muscle stimulation. This principle allows quantification of the maximal voluntary muscle activation.

Provided that the subjects' generated force is greater than or equal to 25% of the MVC, additional forces produced by the superimposed electrical stimulation are assumed to have a linear relationship to the voluntarily elicited initial force [14], [15]. We performed linear extrapolation at 5 twitch torque levels (25%, 50%, 75%, 90%, and 100% of MVC force) to the twitch torque intercept at theoretical muscle relaxation. The VA was defined as [16], [17], [18]

We used a Dantec Counterpoint MK II (Dantec Dynamics A/S, Skovlunde, Denmark) constant current stimulator to apply single, square-wave stimuli with 100-mA amplitude and 500-μs duration. Dedicated software developed at our institution was used for data acquisition and for highly sensitive, automated twitch detection as described by Hales and Gandevia [19]. The sensitivity and reliability of this method have been investigated earlier [15], [16], [19].

Results 

The results of the measurement of MVC, VA of the quadriceps femoris muscle, and the Lewis Score of both legs of the patients are summarized in Table 1.

Table 1. MVC and VA of the Quadriceps Femoris Muscle and the Lewis Score of Both Operated Sides in the Patients

		Midvastus Approach			Subvastus Approach
		Preoperative	3 mo Postoperative	6 mo Postoperative	Preoperative	3 mo Postoperative	6 mo Postoperative
	MVC (Nm)	66.42 ± 21.1	61.12 ± 21.99	69.81 ± 22.91	68.3 ± 20.94	63.11 ± 19.66	71.07 ± 20.21
	VA (%)	81.94 ± 10.47	76.09 ± 19.98	76.79 ± 17.64	76.29 ± 12.73	76.69 ± 14.50	78.23 ± 19.00
	Lewis Score	10.2 ± 4.9	4.7 ± 2.9	2.4⁎ ± 1.4	10.8 ± 5.1	4.6 ± 2.4	3.6⁎ ± 2.4

Data are given as mean ± SD.

⁎ Differs with respect to corresponding value at P ≤ .05.

Preoperatively, patients' MVC (P = .66), VA (P = .06), and Lewis Score (P = .48) between both sides did not differ significantly. Of the 20 patients, 2 had a right-to-left difference of VA of more than 30%. Unfortunately, both patients were randomized in such a way that the leg with the smaller VA was operated using the subvastus approach. Consequently, the average VA in the subvastus group was slightly smaller than the VA in the midvastus group. But nonetheless, the difference was not statistically significant.

There was no main effect of approach on MVC (F = 0.5, P = .49), whereas there was a main effect of time (F = 20.6, P < .001). MVC measurements on both sides obtained at the 6-month interval were significantly higher than measurements obtained at the 3-month interval, which would suggest that the patients had improved knee extensor strength. The approach by time interaction was not significant, which means that we found no difference in the MVC between the approaches during the follow up (F = 0.041, P = .84).

We found no main effect of approach on VA (F = 0.104, P = .75) and no main effect of time on VA (F = 0.205, P = .656). Furthermore, the approach by time interaction was not significant (F = 0.031, P = .863).

There was a main effect of the time factor on the pain assessment revealed by the Lewis Score (F = 11.676, P = .003). The postoperative knee pain decreased within 3 months after surgery on both sides (P < .01). We found no difference in knee pain between the approaches 3 months after surgery. The approach by time interaction was significant for the Lewis Score (F = 7.09, P = .003). From 3 to 6 months after surgery, the knee pain decreased furthermore (P < .01), and, in contrast, 6 months after surgery, knee pain was significantly lower for the midvastus group (P = .02) when compared with the subvastus group.

Discussion 

This study investigated the influence of 2 different approaches, the subvastus and midvastus approaches, on bilateral isometric MVC force, VA of the quadriceps femoris muscle, and the knee pain in patients after bilateral simultaneous TKA. Results were compared with the values of the contralateral leg, and thus, the present study has the advantage in that each patient served as his own control and a true evaluation of the 2 approaches could be obtained. We used a sensitive variant of the twitch-interpolation technique, which is an established, reliable method for the evaluation of muscle function in different joints [14], [16], [19], [20], [21], [22].

To our knowledge, no previous studies were undertaken to investigate the influence of different approaches in knee surgery on neuromuscular function of the quadriceps femoris muscle between legs in individuals with bilateral TKA.

The midvastus approach, which was initially described by Engh et al [11], is a modification of the medial parapatellar approach and provides a good exposure to the knee joint. Using midvastus approach, the fibers of the vastus medialis are split obliquely, thus disrupting less of the extensor mechanism compared with the parapatellar approach. Some authors showed a better outcome in the early postoperative period when compared with the parapatellar approach [8], [23]. However, a potential damage to the neural and vascular structures due to the incision in the vastus medialis muscle could not be excluded [11].

Another alternative to the parapatellar approach in TKA is the subvastus approach, which may preserve the extensor mechanism. Using the subvastus approach through the medial intermuscular septum, the innervation of the vastus medialis is preserved, and the extensor mechanism is not disrupted. Some studies could demonstrate a lower incidence of complication related to the patella contributed to the intact extensor mechanism above the patella. It was stated that fewer lateral releases were required [24]. Moreover, Faure et al [6] could demonstrate a faster regain of quadriceps femoris muscle strength in comparison with the parapatellar approach. Further advantages may be reduced analgetics requirements and a shorter hospital stay because of an earlier return to function [25], [26], [27], [28]. However, it has been previously suggested that the subvastus approach should be avoided in obese patients, in severe flexion contracture, or heavily muscle thighs because of insufficient exposure of the knee [24], [25]. Our hypotheses that VA and quadriceps strength recover faster using the subvastus approach is based on the theoretical advantages that this approach provides a minimal disruption of the extensor mechanism. However, the present study could not demonstrate differences between the 2 approaches at 3 and 6 months after TKA with regard to quadriceps femoris muscle strength and VA.

Similar to our results, Faure et al [6] showed no strength differences at the 3-month interval between the subvastus and parapatellar approaches. Although no strength differences existed between limbs, we found strength differences over time. Knee extensor strength improved from 3 to 6 months after surgery. These results have important practical implications. The rehabilitation up to 3 months after TKA is frequently ignored because the patient has a sufficient pain relief and is able to walk and climb stairs within 12 weeks. Our results underline the importance of long-term physical therapy programs after TKA to restore knee joint function and to improve muscle strength, especially of the quadriceps femoris muscle, because the present study demonstrates that a full physiological and functional recovery usually requires more than 3 months.

With respect to the VA of the quadriceps femoris muscle, we found no difference between the 2 approaches during the follow up. This indicates that the subvastus approach might lead to a similar irritation of neuromuscular structures of the extensor mechanism in comparison with the midvastus approach. Performing the subvastus approach, a blunt dissection of the muscle attachments from the intermuscular septum is necessary. Some authors reported about a higher incidence of postoperative hematomas that might be an indication for a potential injury of the vastus medialis [6], [12]. In addition, after further mobilization and lifting, the entire medial muscle is retracted laterally to expose the synovial capsule and the lateral compartment. This results in a high contact pressure and tension force of the vastus medialis and the patellar tendon. We assume that the increased tension and pressure lead to damage of neuromuscular structures, at least comparable to the splitting of the quadriceps muscle using the midvastus approach.

An effective pain management is the focus of the postoperative period after TKA. Aarons et al [29] could demonstrate that the greatest amount of improvement in TKA is seen within 3 to 6 months after surgery. A sufficient pain relief is a prerequisite not only in the early postoperative period of rehabilitation but also in the intermediate postoperative phase for a successful physical therapy based on the patients' cooperation [2]. In the present study, the midvastus approach was superior with respect to postoperative pain 6 months after surgery. We hypothesized that this result is also related to the soft tissue damage in the subvastus arthrotomy by tension and hematoma. This may lead to an irritation of neuromuscular structures, for example, located in the joint capsule, the quadriceps, and patella tendon, and thus leads to a delayed recovery from pain. The faster recovery from pain in the midvastus group may show some advantages in the postoperative rehabilitation and can mean an earlier return to knee function and increased patient satisfaction.

Conclusion 

In conclusion, the subvastus approach for TKA does not provide any advantages compared with the midvastus approach with respect to VA and MVC within a 6-month period after surgery. Furthermore, the midvastus approach caused significantly less pain postoperatively. According to our data together with our experience of a better exposure of the knee using the midvastus approach, the subvastus approach cannot be recommended.